Well, Baron Vladimir had a longer rule than ol' Prince Leto, didn't he?
Sometimes a rotten head needs decapitation, though, to make way for a fresh, strong leader. Harkonnen are united as a house, struggles for leadership amongst the elite are constant but there is never more than a single leader at a time, so the people always know whom they're supposed to be faithful to.
As for ol' Prince Leto, truly he didn't seek power for the sake of power - not his own, at least. For the sake of "Bene" Gesserit, naturally
And even those insinuations aside, only a man blinded with greed or otherwise mentally weak would trade Arrakis for his homeworld, especially while being unpopular amongst the Landsraad. Yeah he got Emperor Padishah's order. Corrina's orders are only as good as his backing in the Landsraad. What would he do, invade Caladan if Leto defied him? He'd have to use his own Sardaukars since I highly doubt any house would choose to take part in such an ill-fated adventure. Not much to gain. Corrina fails, all the better - maybe his dynasty has been around too long... Corrina wins, he gains nothing except for a burnt-out and ruined world - so he fails as well. Defying Emperor's orders wasn't a death warrant - only using nuclear weapons in an act of aggression, building Abominable Intelligences and few other similar things were.
But then again, Gesserit, too, seemed to have pushed Prince Leto to take Arrakis... and he, quite understandably, didn't want to go against the wishes of his good friends...
Alas, the Guild had shown its true colors in the end (they didn't take lightly Landsraad's feeble attempts to undermine their monopoly, it seems...), and thus Paul was able to destroy everything and introduce a thousand years of Gesserit-style, body-over-mind, emotion-over-reason universe of technological regression and religious fanaticism.
(and please keep Holy Promethium in the promethium tanks - I'm debating this for fun, not to start a holy war
