That's a fair point. But unfortunately, there are no strong love rivals not only in this game, but also in other games too. This is a characteristic feature of the genre. The fear of the so-called NTR is so strong that it can hit the business of developers.
Just go to the Midnight Paradise thread and read what they write about the author after an unexpected "NTR" plot twist . There are games that successfully use NTR elements for the plot, which makes the games much more mature, but their popularity is quite low. The reason for the popularity of DPC and similar games is primarily that he gives what the player wants. The player wants to be loved by all the girls for no reason and doesn't want to know who one of the girls was with before him. DPC removed this by mentioning it in passing.
When I first started playing, I saw in Chad a serious opponent, but in the end he turned into a caricature and sentimental jock. I am sure that, for example, James, Jill's friend Jonathan, Josy's ex-boyfriend and others, if they appear on the horizon, will also be the same caricatured and inconsequential. He loves drama, but this drama always goes the easy way, without significant kinks. You may remember Acting Lessons, but after a ton of criticism, he definitely changed his approach and we won't see such dubious plots with a fire anymore.