I think this really depends on implementation. Paying for bugfixes? No, thanks. Paying for a major version bump like new ms office where you don't really need to upgrade and will be supported anyway? Sure.
Dealing with all that in many cases takes more resources than introducing new features, but, just like you, people expect free compatibility updates.
The deprecation cycle is a decade or more, and in every case there's a paved migration path (GC->ARC, QTKit->AV Foundation, WebView is trickier but for good technical reasons). This strategy and timeframe seems eminently responsible.
Windows 10 will likely support your apps for at least a decade. There's choice.
(I'm using Mac for work. I'm likely migrating to another system instead of Catalina)
If you are growing your market, you can make your app a flat charge. If you are maintaining and and expanding an app for an existing user base, subscription is a better way to go (and something Apple does support well).