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.