The music scene in Iceland is very incestuous, for lack of a better term. Iceland is full of incredible musicians, including incredible drummers, so they will have no problem filling his role even though it just won't be quite the same. It's possible that Jonsi and Georg could continue Sigur Ros as a duo and bring in contract musicians as they have in the past for other roles such as cello, violin, second guitar, etc. I don't think that they will bring in another permanent drummer.
If Orri is able to clear his name, then perhaps he will be able to join the band again. I'm not sure how a large part of the public would perceive that though, since it's really difficult to clear or prove accusations one way or another.
The whole situation is just shit. It's shit if he did it and it's still shit if he didn't do it. Either way, it changes things, which I'm really bummed about.
I applaud him for stepping down and leaving the band out of it.