Well, if we're going to play the mental game of whether we as "executives" would recast...All I can say is that, had I been an executive in charge, I would have wanted the original crew back for a few reasons.
I could be wrong, but I think part of the appeal that exists in recasting young is the longevity of the franchise. (Which, in hindsight wouldn't have been the advantage it at first appears. The original crew performed ably until 1991.)
It also, in effect, assumes that Star Trek had to become a successful franchise, and that was by no means a certainty. At the time, there was no franchise - only a 3 season TV show with a small, intensely loyal fanbase. You don't know - and it would be fallacy to assume - that there would be ST:II, III, etc.
These actors weren't yet able to command high salaries. Your lead star was taking any job that would come along. Nimoy was doing TV movies. Most of the rest of the cast didn't have much of a career at that point. The fanbase is going to the theatre in part to see the original cast.
Paramount (and by extension, me as an executive) would be hoping simply to ride the coattails of Star Wars as long as it could. What are you saving by recasting? You aren't saving money, and you run the risk of lowering the box office return (even slightly).
Finally, I think the idea of recasting is also born out of looking at the question through the prism of today's media market that caters excessively to the under-30 crowd. This hyperfocus on that age group didn't exist.