That's a good point but the game still has 10m subscribers, you'd think Blizzard would be concerned about ensuring as many stay as possible. Considering there are at least 2-3 more expansions on the way, including the upcoming one which is rumored to be huge, it's safe to say the game is still a major priority for Blizzard. My problem is that they've kind of stomped their blueprint into the ground causing a lot of the older players to lose interest; why not try to breath life into the game? The next expansion has a new class which is definitely a welcome addition, I just hope we get more interesting content that isn't just the same thing over again.
This comes to one of my complaints in general about Blizzard and how they support their games after release. SC2 has been out for more than a year yet lacks many basic support features that WCIII had at launch. The chat UI is still archaic, you can't create clans, you can't create tournaments on BNet, custom maps aren't supported as well as promised, etc. Blizzard's approach to the game seems to be based purely on balance for 1v1, and of course there's debate over whether that has been successful. But everything else, from team games to custom map design, has pretty much been ignored.
Instead of supporting the game, they're holding gamers ransom: many of these features which should have been out by now will most likely be included in the expansions. The community is already beginning to slow down as MOBA games get more popular, I fear this will only continue as Blizzard ignores everything except 1v1 (which to be fair, is the only matchup I play religiously - but there are many folks who love team games).