I can't easily imagine the libDems going for a leader saying 'Another leader would have to come in and just say "look, we fucked up, lawd knows what Cleggo was thinking"' and then leading them to the political wilderness again.
They have 5 Cabinet Ministers! 5 Cabinet Ministers!
When did they anticipate getting that number, ever?
Power, even when it is in diluted form, is pretty intoxicating. When Cameron is out-of-the-country, Clegg is the de facto Prime Minister and leader of HM's Government. Read it again, Clegg, a LibDem...leader of the Her Majesty's government. If there is a crisis, he will be the one in front of the cameras, he's the one on the phone to Obama etc.
From what I understand the LibDems and Tories are working well at Ministerial level, and ultimately it is never going to be a 50/50% split, because the LibDems did worse at the election than they anticipated.
Both sides have something to contribute; the tories for their hard-headiness and simple acceptance of reality (the countries buggered, Labour spent all the money) and the LibDems to pull them up and say 'you can't do that' when tory loonies try to gain a foothold, whilst having a few of their ideas actually make it into legislation.
But the Tories will gain a sliver of social conscience and the LibDems will gain experience of national government from the coalition.
But I'll never vote for a party again that looks like it will win an overall majority. Coalitions are the way of the future (works fine in the rest of Europe) and overall majority parties are too tempted by the extremes of power - such as New Labour's obsessions with curtailing civil liberties since 1997.