Even if all browsers gain universal support for HTML5, we must remember that Flash can exist outside browsers. A lot of flash games are in circulation which are simply embedded inside Office documents. The actual Flash standalone runtime is only a few megabytes in size. Flash content is highly compressed and very efficiently packed.
In other words, the problem is not Flash per se, it is the Adobe runtime of Flash. Just like Google changed the browser game by introducing an extremely efficient browser, someone can (in theory) develop a mobile friendly, efficient (and open source) runtime. Flash format quite close to the ideal for games.