Microsoft failed at keeping the surface area as small as possible. When their surface area expanded to a hack to deal with Adobe's hack of replacing code behind Microsoft's back, they went into very dangerous territory.
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.
Not a chance. People will blame Microsoft, because WhateverIUse worked perfectly before. The only parameter which changed is that they updated the OS, therefore the OS is to blame.
The flip side if Microsoft had bitten that painful bullet up front, then I believe that going forward companies like Adobe would have learned to not try to hack the system like they had. That would have given everyone, including Microsoft, a better reputation.
But of course Microsoft did not do that. And now Microsoft has such a history of nasty bugs that, even if Microsoft is in the right, nobody is going to believe it. (Which of course locks Microsoft into its current strategy of bending over backwards to accommodate everyone else's bugs, causing more bugs...)