This is why Adobe really needs to open-source the Flash Player. What do they have to lose from doing that at this point? The player only stands to improve, an improved player drives adoption, and with the install base currently in decline, I think they might want to consider all their options.
I'm interested to hear where you've read that the installations of the Flash player are in decline. It seems to me that when all the browsers can fully use all the HTML5, CSS3 fun you will still be able to use the Flash plugin as well and the install numbers won't decrease anytime soon. Won't these technologies just co-exist?