[DISCLAIMER] MAJOR SPOILERS AHEAD!!!
READ ONLY IF YOU HAVE FINISHED WATCHING THE DRAMA

I binge watched the whole drama in 3 days. With a unique and fast paced plot, it felt like riding a roller coaster. The whole cast did a marvelous job, especially SJK as the grandson and the magnificent LSM as the grandfather. That duo was everything for me, how their relationship progressed from suspicion, to enemosity, to camaraderie, to trust was a work of art.

Now I wish to discuss the main issue at hand, which is the plot itself.
While the fast paced first episode pulled me in, the fantasy element in ep2 initially put me off (my mistake for not checking out the original work's synopsis first and not taking the title literally), but eventually we come to accept the fact that our hero HW had died and was reborn (for some reason midway) as DJ; the grandson of the soonyang family, the owners of the corporation he had previously worked at as HW. We also soon learn that the person he was reborn as, the youngest grandson DJ, had been dead and long forgotten by the time we saw HW working at Soonyang in 2022.
Although at a loss as to why and how he was reborn as DJ (just as much as we are), he accepts the truth (too swiftly tbh) and soon finds a purpose in this newfound life he was bestowed; he decides to take revenge on the Soonyang family that were behind his murder as HW, while also trying to salvage his poor family and save his mother from her untimely death.
First, He has to fight his way towards success and power in order to establish a place for himself in the family that he was initially not considered a part of (his father is an illegitimate son of the founder, who married an actress against his father's wishes and got disowned). He uses his knowledge of future events and the mental capabilities and expertise he acquired in his previous life, to one-by-one beat down all the evil and corrupt members of the Soonyang family, in order to reach his ultimate goal of becoming the chairman of the company, a promise disguised as a warning that he had made early on to his grandfather and the founder of soonyang.

So, in essence the story at first seems to depict an elaborate, smart and meticulously plotted revenge. He chooses to squash the business of the evil and corrupt members of his family, whom he knows one of which had orchestrated (in the future to come) his murder as HW and most probably had been behind DJ's death.
We are also shown clearly, that despite DJ's ability to utilize his knowledge of the future to change certain outcomes in the present, it is clearly established that when it comes to matters of life and death, there is nothing he can do, and no matter how he manages to change details in people's lives, ones who were destined to perish will ultimately perish (like HW's mother). So by this point, any rational viewer would realize that DJ as well is bound to meet his demise sooner or later. And despite an initial failed attempt at his life, I for one knew I would be disappointed if the drama ended with him alive.
So for that reason, I was not shocked or angry like others seem to have been when the writers opted to show him getting killed. On the contrary, that was one thing that made sense to me (much more than the mechanics behind his reincarnation into DJ or his shifting back to HW did). In all honesty, somewhere in my brain that was already exercising a major amount of suspension of logic, I somehow had the feeling that the drama will choose to use the card of "HW hadn't died", and I foresaw the possibilites that he might as well die as DJ as destined to happen, and return to his previous/future self as HW.
And that was exactly how it unfolded, but with a major plot twist. A twist that helped answer a question that must have been on everyone's minds since ep 1; namely the unrevealed background story of how a impoverished high school graduate managed to climb his way up to become a team manager and the trusted right hand man of the chairman of a large corporation, it managed to provide a convincing albeit sad, dark and unexpected answer. That exact same twist, served also as a kind of hint as to the important question that we as viewers had chosen to push to the back of our minds all through the drama; none other than why he was reborn as DJ in the first place. The "how" is impossible and futile to try to wrap your head around at that point, but "why" he, HW was reliving his life as DJ, was a question that certainly needed answering.
In that sense, the twist that was revealed at the end of ep 15 although shocking and somewhat disappointing made absolute sense, and neatly tied all the loose ends. HW was partly responsible for DJ's demise. We had also been shown at that point brief flashbacks of HW's miserable life at the time, paving the way to our understanding of how he was pushed into silence and obedience, and why he chose to do that against his conscience in order to secure himself a stable income that would help pull him and his family out of misery and poverty, living for 20 years serving his masters, the same people that he always had known were capable of the most heinous of deeds, without questioning their orders or judging it. One can say that he kind of brainwashed himself into becoming their ever-so loyal servant and slave.
Having a rego at life as DJ, he thought, his purpose lied in taking revenge. But he ultimately found out that it was all for repentance. He needed to right the wrongs that were done to him, both as HW and DJ, and hold accountable the people responsible. He was reborn as DJ to gain insight into the person he had witnessed getting killed with his own eyes and chose instead to erase all memory of his existence and untimely death. He was reborn as DJ, to learn about the machinations of the chaebol family he once served, to achieve the things he had long craved for; a higher education and a stable family life. But although he was focused and determined to prove himself and make the most out of the miracle that is him being reborn as a chaebol family's youngest grandson, he must have known that no matter how much he tried, he could never alter his destiny.
Logistically and logically speaking, him switching back to HW at first felt ridiculous. How come a week-long coma could enable someone to experience 17 years worth of life? But, seeing the events that unfolded, how HW chose to give justice to DJ's wrongful murder, while also ridding his grandfather's legacy, that is soonyang, of the corrupt and evil family that almost but all destroyed it. But, how the writers chose to achieve that feat was somewhat laughable imo (check the finale's recorded phone call, a conspirator for murder who forgets to end a call, and address his criminal mastermind boss who comically names himself for the audience LOL), and although that kind of dampened my enthusiasm in the plot that up to that point was neatly and tightly woven for the most part, setting my frustration aside and focusing on the outcome, that is HW took revenge for himself both as HW and DJ and that although DJ was tragically killed by his kin, his death wasn't totally in vain, soonyang was rid of the poisonous venom that was choking it.

On a side note:
I learned after watching the final episode, that the original web novel has a totally different ending, what could be considered a conventional happy ending. Some people might be enraged as to why the writer of the drama chose to change the ending in such a drastic way instead of simply adapting the same ending. I can not speak with certainty since I haven't read the original, but as a rule, I am of the firm belief that if a story that has a fantasy or sci-fi element chooses to establish certain terms and codes, it MUST stick to it. And since Reborn Rich had chosen to (rightly) establish that altering people's death is out of the question, it only makes sense that they stick to their rule, however much it might upset the fans.
Watching the final couple of episodes, I had a suggestion to how differently the story could have been written. they could have instead chosen to adopt a parallel universe scenario (one which was almost alluded to near the end with a side to side shot of HW and DJ) ie, that two peoples' souls swapped into each other in two different universes. My suggestion although a bit confusing would have been then that they make another actor play the role of HW after our lead was reborn as DJ, and when he returned as HW they show DJ as the same actor in flashbacks. I know Some might laugh at this, but look me in the eye and tell me you were not bothered by the fact that no one recognized HW's face as that of DJ. *side eye*