The real reason it'll not be much of a problem is that the current Flash 11.2 plugins won't spontaneously disappear when the next version gets released. Indeed they'll have 5 more years of security updates.
So the only thing non-Chrome-using Linux users will miss out on are things that use the new features of Flash introduced after that date.
That makes sense. I was starting to wonder if this was the end of flash for Mozilla on major websites like YouTube, but 5 years is a really long time for support. Hopefully, we'll see an emergence of widespread html5 use by then and not have to worry about having proprietary web technologies working.
So the only thing non-Chrome-using Linux users will miss out on are things that use the new features of Flash introduced after that date.