To add to the list of reasonable reasons for discontinuing products, there's also the question of relevance. If WebM support is added to some future IE version (not out of the question if WebRTC in its current form catches on), will the WebM plugin get added to that list? That's exactly what happened with things like Google Desktop, which I used to love but became completely superfluous when windows added a decent file search. Not everything needs to last forever, especially for desktop utilities and libraries, where the most useful tend to be folded into the OS itself over time (see: "sherlocked").