DISCLAIMER: this is all my opinion. Please disagree all you want. But given I was asked, I am explaining why I think it is a bad idea.
Ok, let me paraphrase that so that you can see what I meant more clearly.
Your argument is valid if there is not limitation on resources. If, for example, DB could create any possible scenario, then sure, let include one simply for the reason of people that enjoy it taking. In fact I would also enjoy an scenario in which MC deviates from the sacred Timeline and he is captured from the Time Variant Authority. If there is an Alligator Loki, I want a Koala Emma*, given the way she hugs MC in one of the images.
The idea of a harem is very hard to pull consistently, without violating character's internal behavior. Here what you are saying is essentially that Charlotte, a woman that has suffered from the abused of multiple man, would be willing to share the same man with her two children and her sister in a romantic way. That Sucy, that has showed pure hatred towards MC (this could change though) would be willing to have her sister, the one she protects above all, date a guy that has to split himself between 4 other persons. Emma, that for all I love her is essentially a self centered child that goes to the point of justifying sex of her love interest as a manipulation made by a woman to rationalize not being number one in her lover's eyes would be ok to share MC among any other. And Lucy, that has always felt jealousy against her sister would forever feel she is simply being number two.
Of course, you could try and do something in which all this different personalities would learn to love and like such an scenario and all live happily ever after. But that is not consistent behavior.
Truth is, Charlotte will always find it weird that a mid thirties (my guess) guy is doing her extremely innocent daughter, imagine if he is also fucking her. Lucy will most likely never be able to stop feeling like an additional wheel if she was in a relationship with MC and her sister, given her inferiority complex. Suzy's personality would have to go from pure rage to a puppy that accepts everything and Emma would need a lot of maturing to understand that she would not be the center of attention but simply one from a group.
But realistically, it is an almost impossible scenario to pull in a way that would fit the rest of the story. This is not a comedy, not a parody, not a fuck fest in which MC's cock is god given. This novel is (obviously my opinion but I would guess is a fairly educated guess) is trying to go for a more serious dramatic approach.
It is also the kind of thing in which most choices are almost irrelevant, as they are not real choices. For a choice to be real you have to give up when you make a choice. If you can have everything, then why make the choice to have less than everything? Choices like that are not real options but simply a right answer and a wrong answer. If you want, a choice is only as good as what you leave behind. So having a harem, in which you are choosing all girls, also means you are choosing no one. It is really the most pointless decision in the game.
So given that DB still have to advance a lot the plot and that you have, on the very least 4 routes to make, I can only see a harem route as a waste of time and effort for something that would not fit the rest of the story. It is simply pure fan service made to inflate the reader's ego about how phenomenal they are that all love interest are willing to share him. They are fun to have? yes, but they are really a joke route most of the time in any more serious novel.
There is a lot of novels that are made simply to go and try to make a harem. But I personally don't see how a harem setting will work here.
DISCLAIMER 2: I do enjoy harem in games in which it make sense, if you have not played it play Once in a Lifetime. It is complete and it is fucking awesome! And the harem setting makes a lot of sense given the overall mood of the novel.
Edit: highlighted the most important part of this wall of text.
*Bunny, kitten or puppy also works
