I don'y think it would be fair to write off anyone who was a coalition minister. The LibDems are strongly in favour of PR, which tends to lead to coalitions. In 2010 most party members would have preferred a coalition with Labour, but the numbers did not add up and so the difficult decision was made to work with the Conservatives.
The LibDems put country before party, something which no other party has done. Norman Lamb, Vince Cable, Jo Swinson, Ed Davey and others did a good job. They should most certainly not be disqualified from leading the party.