W.R.T. compositing, browsers took a long time to move to even just GPU based compositing, and full GPU based rendering took longer still. Certainly, if Flash hadn't been killed off so prematurely it seems reasonable that either they'd have done the work to keep up, or users would have organically abandoned the platform.
Indeed, you make a good point that it was really a fight over forcing devs to use Objective C and nothing else. Flash was just roadkill in that fight.
It took a few years for plain old HTML/JS sites to be optimized for mobile, and not many of them were then. Given some time, Flash devs would have too. I'd already optimized my own and was optimizing a Flash-based gaming site for mobile when Apple pulled the plug.