The game is still very much alive and well. I watch StarCraft II constantly and there are tournaments on all the time. It will contract if there isn't anything fresh added into the game, but it's solid. It's just not as popular as the online arena games like LoL and Dota2, but it's solid.
Legacy of the Void is a much needed thing though and adds some fresh changes that I find very enjoyable. It's not that Heart of the Swarm is bad. It's actually very well balanced. It's just that I think HOTS has some hard counters built into the game that make certain compositions completely ineffective in a tournament/league setting. There are units and compositions that you just *don't* see used, like ever. LotV hopes to address some of that, as well as speed up the very early game where you're just mining and building stuff.
Also, SC2 is larger than Brood War ever was.