Oddly enough, MacOS had better back-compatibility with the old applications than the newer ones. It seemed like pretty much every app broke at least once between Systems 7 and 9, even while the 1985 apps ran fine. (Perhaps the developers were being trickier in how they abused the OS.)

Incidentally, I think this figured into Apple's thinking regarding limited back-compat. They had already unintentionally forced a number of application upgrades, why not do something positive like move everyone to a new OS/CPU/API in the process?