Hypothetical question:
If the Lib Dems had never made their tuition fee pledge, but had still served in the coalition, would they be in the same or a similar state to the one they are in now?
Put another way, which factor is more responsible for the complete irrelevance of the Lib Dem party? Selling out their voters with this U Turn, or getting into bed with the Tories?
Could either one of these things on their own have caused all the damage, or did it take both?
I think it's an intriguing question. There's probably a solid line of argument that the Lib Dems were at least partially fecking themselves over the moment they got into government, but then I don't think they ever helped themselves. And I mean that in a way that goes far beyond tuition fees.
For what it's worth, even as a staunch anti-Tory person, I don't think the Lib Dems were necessarily wrong to go into the coalition on principle. They were kingmakers and ultimately the Tories were the only major party they could work with. From a pragmatic POV, being able to neuter their worst aspects while implementing progressive policies isn't a bad goal.
But the problem is, it's difficult to see what they implemented that was particularly substantive. There were a lot of decent minor policies, but ultimately any party of government is judged on their flagship policies and on certain key moments. The Lib Dems were, for the most part, fairly keen on Tory austerity, and only wanted to water it down slightly.
They should've demanded voting reform of some kind. This is one of their proper flagship policies that sets them apart from the big two, and would've been a way to guarantee continuing influence. The referendum shouldn't have been on whether or not we implemented AV - it should've been on what kind of voting reform we were implementing. Or they should've pushed further to demand outright PR from the start. This should've been something they were willing to sink the government over. They weren't a tiny junior partner like the DUP propping up a party just short of a majority - they garnered over 20% of the vote (around 2/3 of what the Tories got) and had around 1/6 of the coalition's seats, if I remember correctly. They should've demanded one of the big four offices of state - Cameron and Osborne kind of came as a package, so they should've insisted upon one of FS or HS. Tories may have spit the dummy out of the pram but it'd have exposed the myth of Cameron and his party being a reformed moderate Tory party, and would've again demonstrated the Lib Dems had red lines they would stick by. In the end all they got was the fairly meaningless Deputy PM position. The Tories took them for an absolute ride.
So, yeah, a lot was going against the Lib Dems, but they didn't do themselves any favours. And their branding was dreadful. In 2015 I can't remember them really promoting anything they'd done all that successfully. And their arguments about neutering the Tories seem fairly laughable in retrospect when you see what's happened since.
But I arguably think 2017 was an even bigger missed opportunity for them. They were never going to get to 50+ seats again, but a better campaign would've perhaps gotten them 10-15% of the vote, and maybe 20-30 seats. Instead they allowed themselves to be outflanked on gay marriage by Theresa fecking May, when they're meant to typically be more socially progressive than Labour.