Blizzard isn't going anywhere. In 10 years I'll still be able to log into Battlenet and download Diablo 3 and play. Much the same way I shop for physical goods, I tend to go with manufacturers with a long solid history.
"isn't going anywhere" isn't really the operative question though. Microsoft certainly wasn't "going anywhere", and it didn't stop them from just shutting down the entire MSN Music Store (their attempt at competing with iTunes), stranding all of those nice, convenient digital-only downloaded music files on whatever device they happened to be on at the time, never to be moved or re-installed/re-downloaded ever again, because the authentication servers went away without the *need* for them being removed.
It's a smaller subset of the issue, but publishers like EA and Activision aren't "going anywhere" but it's not stopping them from taking down multiplayer servers for games less than two years old in some cases - and it's an easy extrapolation to see them taking down authentication servers for something they don't feel like supporting anymore.
To which the digital adherents will say, "But Microsoft and EA are evil, while Valve and Blizzard are good." All fine and good if you're comfortable with trusting that they will *always* be good, and there will never be a business reason to change their strategy WRT this issue, or a change in leadership who *wants* to change their strategy WRT this issue, etc.
I for one do not have that level of trust, given the steady erosion of consumer rights WRT First Sale and Fair Use. Buying digital is akin to relinquishing all of those rights that one would, by law, enjoy when one buys physical media (paper, CD, DVD, etc.) and doing so intentionally. And even old-school curmudgeons like me have no argument with that arrangement on its own - we wouldn't expect those rights on a digital purchase, and for bite-size games on an iOS device or a fire-sale price on Steam (Deus Ex:HR for $8 for example) we don't have any heartburn over giving up those rights (if I may be so bold as to generalize for the other curmudgeons). But it's a different story on full-price, AAA, no-going-back, chain-the-disc-to-your-ankle-with-Steamworks games.