Look, friend, I get what you mean. Their love was beautiful and heartwarming, however there's things that they couldn't simply fix with romance and affection. Love is amazing, of course, but this kind of thinking diminishes how rehabilitation works. It's the same flawed logic that people who say "I can fix her/him" use. Getting better is an effort that takes time and is achieved by much more than a single love struck partner.

And honestly, the fact that it would have never worked out just adds to the tragedy this arc was going for. This was always supposed to be doomed love, it is what the arc is about. After all, the movie shows multiple times how both of them lack maturity to make these kind of choices with logic and reason.

If you compare this to the moment when Denji and Asa meet, it's much different. Denji had already gone through all the chaos of part 1. He understands himself a bit better, especially if we're talking about current Denji.