He did hold the line at electoral reform, we had a referendum on changing to AV. The public overwhelmingly voted to stick with FPTP.
Granted AV is pretty crap and complicated and some form of PR would of been much better.
I'm fairness to clegg and 2010, what could they do? Not join a coalition and leave a weak minority Tory government in power and find that absolutely nothing gets done, as the country crashes into further debt and recession. Or join in with the Tories hold them back from there worst policies manage to enact some of your own policies and actually be a part of easily the best government of the last 14 years.
By 2015 the country was in a much better place than it was in 2010 there was a decent fiscal recovery happening, then kicked the libs into touch gave the Tories a couple of majorities, voted for Brexit, left the EU and utterly screwed ourselves over all while labour was attempting it's left socialism phase that got them absolutely nowhere.
If only we had stuck with the lib dems somewhat, maybe we could have avoided the chaos of the last 8 years.