Earlier today, I cam across a post in my RSS feeds from one Benjamin Kerensa, titled "Can we kill Adobe Flash?"
The problem with such a move is that, by killing flash support in one fell swoop, quite a bit of the "old web" becomes unusable. Old flash animations and games become inaccessible, and many users would revolt by simply using old and insecure browsers. Just look at how much pain there was with bringing people away from Windows XP, and IE 6.
Unless Mozilla—or a combination of browser vendors—release a tool to convert flash applets to HTML5 with few issues, such a change would simply anger users, and potentially turn them against the open web.
It also does not help that browser support for various tools needed to emulate a Java-like or Flash-like development environment range from "fragmented" to "non-existent". With both Java and Flash, there is a common ground—the Flash player, and Java JDK/JRE—which they can learn, and expect that all systems will run their code the same way.
The primary reason that many flash animators (that I know) refuse to switch to HTML5 is due to the lack of tools that make it easy for them to simply create their animations without any need for coding. As they would say, "I don't wanna code. I just wanna make my art." We need something akin to the Flash creation suite before HTML5 would become an option for them.
Unity came close to making a great platform for games, but HTML5 lacks some of the tools that their NPAPI tool has—locking mouse cursor to a window in a first-person shooter, for example. With NPAPI support ending soon, Unity is going to be hurting, and that will hurt adoption of the web as a platform. People will not turn to the web for publishing their games, but will instead turn to desktop and mobile applications.
Edit: I would also like to add that this idea of "force change or do nothing" has become so annoying, that I can see why more and more users are beginning to dislike and distrust developers, and why many more are abandoning the idea of becoming one. This mentality strikes me as anti-"open web", not pro-"open web". That, in my eyes, is a severe insult to the mission which Mozilla holds so dear to its heart.