I think its way too soon to say who will be directing future ST or SW movies until the ones that JJ have been contracted to do have been made.
Afterall, the proof is in the pudding. If these movies fail I bet both companies will find a way to release him from these contracts.
I would like say, though, if JJ is doing all these new, hot movies, that doesn't leave any room for any up and coming directors. Which is a pity. Being a creative all my life, I have always believed in giving new talent a chance...it's only fair.
And second, if I was Paramount and I found out what JJ was pulling, he'd be fired plain and simple. Working on these two movies is a conflict of interest and I, as a producer, would feel betrayed. It's about ethics. I'd even do it to Spielberg if did this to me.