Flash Plugin is retiring at the end of next year, i.e. the end of 2020. It should be usable up until then as browser vendors seem to be supporting the timing. Not much time but you could in theory make something.
There are other reasons though not to consider the web. Casual gaming is much more on mobile. Rather than play Candy Crush on Facebook or King over lunch you can do it on your phone.
For mobile you can use Starling to make an AIR app, which uses OpenGL under the hood to access the hardware on Android. Apple used to use OpenGL but switched to Metal a few years ago so AIR targets that instead.