I do see your point. Apple will have a hard time if they switch a very popular one time fee app to a subscription model. Then again, they're charging monthly for FCP for iPad....Nope, and I doubt I ever will. But this is different. They’ve been providing 13 years of updates to fcp for free, I don’t see them charging. People would move to more popular and industry standard solutions.
The only real alternative would be Resolve. I am more curious about what they do with the Pixelmator apps