Honestly I think both Pep and Mou aren't long term trainers. What makes you think Mourinho will build you more success for the future? It's definitely not his track record, since he's never stayed for more than a few (3?) seasons at any of his clubs. Same with Pep, even though he's successful where he goes, he leaves after a few years, either needing a sabbatical (being an insane person is tiring I guess) or a change of scenery.
That being said, I don't think having the same manager for twenty years is a prerequisite for success, look at Bayern, Barcelona and Real, they switch managers quite often and seem to do well. The staff behind the manager DoF, TD and what not should be stable, the guy shouting at the lads from the sidelines can switch up every few years just fine. The lads themselves change all the time too, so why can't the trainer?