It completely prevents passive surveillance, however. Sure, you can MITM, but the point is now to look at anything you have to do an MITM attack, unlike now, where most traffic is unencrypted and you can do surveillance passively. This makes surveillance more difficult. Net gain for everyone.
I'll concede that passive surveillance would not be possible. However, I think that benefit is marginal - AFAIK, unencrypted support in HTTP2 is optional and won't be supported by browsers in any case.
