My personal suspicion is that the current LD position is the new normal for them.
I understand your logic, but I'm (somewhat obviously) on the other side of the fence.
It's a two stage process for the LDs to really return to strength IMO.
Stage 1 is now - rebuild at a council level, campaign on the big issues, slowly increase the number of MPs the party has. This also means losing far less deposits.
Stage 2 is when there's been a return of quite a lot of councillors, the party is on something like 20 MPs and far less deposits are lost. The party can make a play as the third party and really start putting pressure on the two big parties again.
The LDs are two years into the post-coalition rebuild, but increasing the number of MPs this year was ultimately a good result. So if you peg the next election as a couple of years away, and in that one you get the LDs up to 20 MPs (which is far from impossible - St Ives, Ceredigion, Fife North West and Richmond Park makes 16, so you're already half-way there with negligible Tory->LD swings and Labour voters being more willing to vote tactically) and after that you can start thinking about pulling a 1997.
So I think the current situation as a political observer at Westminster is the new normal until the next election, and maybe until the one afterward. But I can't see the Lib Dems being stuck at some 7% rut forever.