The thing about Chris Pine or any other actor to portray James T. Kirk after William Shatner is that Shatner is just too big of a star at this point and it's hard to imagine anyone else in the role after 40+ years of him strongly-identified with this iconic role.
It's as simple as that.
I know I'm not the first to bring it up, but Sean Connery and Bond applies here. Connery's a big star. While not technically the first actor to take on the role (as the aforementioned case of Barry Nelson), Connery was strongly identified with it. For that matter, he still is. But quite a number of actors have played him since then. Some very well, others, admittedly, not so good. Pine will be faced with essentially the same problem as the post-Connery Bond actors - as will Quinto. I feel Daniel Craig rose to the occasion, and it's possible Pine and Quinto will do the same.
It's equally possible they won't for all I know, but let's wait and see.
