As I said, the series are obviously different. The inclusion of a number of arcs with a dramatic focus within School Rumble means the show ISN'T all "lawl mindless comedy," even if comedy is the primary focus. Had I only the anime to go by, I wouldn't expect a serious ending, but as the manga went on the focus shifted; perhaps not fully, but it did shift.
You said it yourself, the series gradually steered away from a HarimaxTenma end. So why are we left with what we've had all along? Using Maison Ikkoku, as I said before, Mitaka didn't waver until the end. Why is School Rumble magically different, because you decided it was restricted to comedy only, since that was the main focus? Sorry, it doesn't work that way. School Rumble has dramatic elements, and the past few arcs are reason enough to believe that this was going to end with resolution with Harima.