It also doesn't sound all that hard to update your browser.
In essence, upgrades are not as simple as you may think due to forced platform incompatibility/vendor lock-in. Forcing an upgrade like this can cost users a non-trivial sum of money on top of what you're already charging for the service!
So while I appreciate the moving into the future, I feel bad for the people whose wallets are going to feel the pain of such a move.
I recently acquired a Macbook Pro from work, but I'm using Windows 7 on it full-time. I'm far too wary of being forced to upgrade when Apple stops providing security updates to the version of OS X currently installed on it.