Blizzard is a rather successful company.
I'm pretty sure they always look for ways to both retain subscribers and attract new ones.

I'd imagine they will stagger the releases of Starcraft, Diablo, Warcraft and MMO games, along with anything else they bring out.
The MMO games are subscription based, so the free Diablo with a year commitment could easily become their model for any non-subscription product they release; that way, they get a year of subscriptions from anyone interested in both Wow and any other Blizzard product.