It's not as simple as that. If it's just Adobe WhateverIUse that breaks, sure, people will blame Adobe. But if it's Adobe WhateverIUse that breaks AND WordPerfect WhateverSomeoneElseUses and maybe also Borland SomethingSomeoneElseYetAgain uses word will get around that the new operating system "breaks things" and people will stay away from it.

Even if noone will ever use all three of the broken apps.

In this sense, maintaining backwards compatibility is simply a matter of reputation management.