I agree. I also think adding a harem route would be fun. I mean the MC is trying to spread his seed to as many women as possible, so it only makes sense he'd end up with a harem.
He's not trying to spread to as many as possible. He's just trying to make sure it survives. Throughout human history, generally having offspring is the way to do that. The vast majority of the time, you don't need 75 women to ensure your line's survival. Have a few kids and the chances are very high (barring a war or plague or something) that your lineage will continue well into the future.
Sure, the chances go up if you have a lot more children, and having multiple women is one way to do that. There also is IVF (since this game takes place in modern times), so tons of that could also easily ensure a bunch of children. I don't think a harem route would work well in this game at all, TBH. Too many women are either at odds directly with each other, don't get along (even if not actively against others), or have low enough self-esteem that other women would hurt them badly. I see no way a harem could ever work with this story.