Well said, Karin and Smlime16. My thoughts exactly. A one-off reunion would be nice, but I too think the series had run its course and with three of the boys away from home, it would be a stretch to make this work and not retread (derivative, so weaker) story lines.
Older, similar family-oriented series like The Cosby Show and Family Ties tried to plod on by adding new babies or live-in younger cousins, and for instance Home Improvement ended in strife, because the leading actor had enough. Also, you could feel The Adventures of Pete & Pete coming to an end when the dynamic changed as older Pete was about to enter college and younger Pete hit puberty, and could hardly be expected to live in a kind of dreamlike fantasy world. No actor wants to be stereotyped in one role, and I can imagine teenagers hating it when they keep being seen as that cute little kid from years ago.
I really think it's a good thing MITM didn't peter out with embarrassingly weak episodes, which could easily have happened.