Why would they change the cast after in the first place??
People seem to offer reasons like "Oh, they'll recast if the actors they have now really stink up the place." and "They'll recast if the fans don't like the actors." It seems like you're making all kinds of suppositions based on Urban (that's his name by the way) simply stating "I'd like to play McCoy again." That is no way means that they might recast someone else. I don't know why you would make that conclusion. That's just not the way things are done. They all signed a 3-movie contract. If the film does well, he'll play McCoy again for sure. If the film doesn't, he won't. it's not like he's sooooo famous that he'd be too busy to play him again and from all counts the cast had a good time and good chemistry together. My conclusion that maybe you wanted someone else to play McCoy could easily be drawn here. It's not as much of a fanatical stretch as your premise seems to be.
This is really the whole crux of the "recasting argument."
The question is... is a character tied closely to the actor portraying him/her, or not?
For anyone who's not aware of my position... I'm on the record as opposing any ongoing "recast" but I'm on the record as being in favor of a "one time recast" to give us backstory on characters we already know. In other words, I want to see a "where they began" film, but I don't want to see "the 5-year-mission - REDEFINED." Add to the mythology, don't overwrite it.
As far as I'm concerned, Kirk looks, acts, and talks like I've known him to look, act, and talk for 42 years. No "recasting" will ever overwrite that... and I will be STRONGLY DISINCLINED towards accepting any "recasting" that shows us someone dramatically different, but supposedly "the same person." Of course, I'm not the same person today that I was when I was 12, or 17, or 22, or 28... and the further back you go, the less was I like my current self. So... the further backwards we go from the first time we saw the guy, the more acceptable seeing a "slightly different" character will be (in terms of performance and in terms of appearance!)
I'm not totally won over by some of the recasting... hell, I'm annoyed that we're seeing several of these characters at all, since I think that their presence seems contrived and inappropriate... and just "fanboyish." But as long as it doesn't "overwrite" what we already have, I can live with it.
NOW... given that argument ... that what's "set in stone" is Shatner, Nimoy, Kelley, etc... I have NO issues with any casting which honors and represents those same, essentially unchanged characters. And I DO have issues with treating the new cast as "a new, revised canon." However, I'm pretty happy with MOST of the casting. Pine can work, Quinto can work, Urban can work... and I have enough faith in Pegg to think that he can do Scotty justice even if he looks different. As for the others... well, Saldana looks the part, as does Cho, reasonably... Yelchin is a bad choice from an appearance standpoint but who knows how his performance will be?
But none of these people "are" the characters. They're simply representing already-existing characters. So, if three years down the road PPC decides to do another one, it shouldn't matter very much if the cast is entirely different... because the "target" isn't Pine, Quinto, etc... it's Shatner, Nimoy, etc...
I just can't think of anyone better suited to play McCoy or Spock than the guys that they've got, and I have a pretty good feeling about Pine as well. I can imagine Pegg doing the role of Scotty justice. The rest are purely background... "second bananas"... and I couldn't care less who plays them as long as they're doing decent (non-spoof-ish) representations of the folks in the original series.
I doubt that they'll recast any of the roles... IF they do more movies with this cast. But if they have to (over availability or whatever) I seriously don't see any problem with it, as long as they recast based upon the original characters rather than the new-movie versions.