I think Corbyn bought in two kinds of voter young idealists, who are now struggling with some pragmatism, going on the previous discussion Starmer isn't really that much different from the Millibands. But pragmatism isn't that sexy and its a bit of crushing of dreams and its pretty ugly to watch at times.
The Green's are fine for most of that group though but also constitutes a wasted vote.
The LD's use represent a pretty good center-left/center option but Brexit has pretty much killed the party (more so than the coalition ever did) they now represent essentially a very hardcore rejoin EU group who don't realise how they lost with the last election meant its not an option for decades. Meaning they've picked up a lot of One Nation Tories who are the least evil version but when you more ascribe to the SDP side its a shift rightwards whilst becoming one issue. They're coming back by the looks of things but I was part of the Kennedy generation and they've had successfully worse leaders leading to a complete non entity from a lack of choice.
I don't know who is really there to represent the Corbyn left currently if they don't want to be pragmatic. Groups like NIP are complete crackpots.