I am by no means a hardcore TOS fan (Kirk irritates the hell out of me), but why go through all that effort to repackage characters with "hot" new actors?
It would have been just as easy to have cast the same people but as totally new characters with a new ship, etc. Reworking TOS was always going to get the backs up of the series' fans, after which they'd have a fine line to walk, either in retelling old stories with new twists (thus branding the writers/director as being unoriginal), or doing totally new stuff (thus branding the writers/director as having no commitment to the original series).
A fresh new crew would allow them to tell whatever stories they so wished, without the added pressure. Anyways, that's just what I wish they'd done. No going back now.