#5 The introduction of Tuition Fees was the turnkey to @LibDems collapse. This helped Cameron on a seat basis in 2015, but it long-term decoupled 25% of the electorate (disproportionately young) from their political anchoring. By 2017 many of them joined @jeremycorbyn (6/10) pic.twitter.com/yGRT9vytUZ

— James Kanagasooriam (@JamesKanag) April 8, 2019

I think any cooperation between the LibDems and another party would be less than the sum of its parts. They might have some infrastructure that a new party lacks, but they will bring a lot of baggage.
It sort of reminds me of the German Left party, though on reflection the similarities are fleeting.
They were a fusion between a Social Democratic splinter formation from the West and the successor to the Eastern Socialist party. Though the problem here really wasn't the Iron Curtain and the Stasi (as soon as you want to raise taxes on the wealthy all those things become your fault. Remember that Angela Merkel was already politically active in the GDR, which is no blemish on her character, yet the SPD, the only party that didn't absorb part of the old East German apparatus, is always too close to the Reds). Here the problem was that the Eastern party was a mass party that wasn't particularly willing to run radical policies in states they controlled. And that cost them a lot of momentum.

Shrinking in fusions is also something that has happened to most leftwing parties that tried it in recent years. And why not? If you can't make a plausible claim to power, than you can only be hurt by compromising on positions.
But to get back to the point: A party that runs on remaining in the EU, very popular with young people, but also carries responsibility for tuition fees, as popular as cancer among young people, might very well fail to lift off.