I think that's a misunderstanding of what Momentum is. There's certainly a large (generally older CND/Militant bunch) 'Jeremy or bust' camp within Momentum who would see any push for Corbyn to step aside as a betrayal regardless of whether it came from left or right, but these people aren't a majority within Momentum or the Labour Left in general. They're just the people who have the loudest, least nuanced opinions and who therefore get the most attention. Empty cans rattle loudest, as the old adage goes. The vast majority of people I know who are affiliated or active in Momentum are Remainers in their 20s who are capable of holding nuanced opinions and would happily back any 'left' candidate going forwards if Corbyn was to step-down.
There are obvious reasons that no-one on the left is pushing for a change of leader. The elephant in the room is that the media and figures on the right of the party will come at any vaguely left-wing Labour leader just as hard as they've come at Corbyn (they're already coming at young left-wing MPs like Laura Pidcock with the same attack lines they use on Corbyn, despite the fact that she has none of Corbyn's baggage or dodgy associations). Another leadership election would be a couple of months of brutal in-fighting and awful press coverage, after which we'd be back to square one with a left-wing leader being constantly undermined by members of their own party in Parliament. All of this distracting from the Tory-caused crisis we're knee-deep in and with a possible snap election on the way.
I'm as frustrated as anyone with Corbyn's irritating fence-sitting re: Brexit, but at this stage all factions need to look at the bigger picture and pull together instead of flexing their egos, the right of PLP would do well to look to the recent past and realise that had they not thrown their toys out of the pram for two solid years following Corbyn's election, the 2017 election would probably have delivered a Remain majority in parliament instead of a razor-thin Leave one. Boris will be a disaster as PM and Tory leader and if Labour can't manage not to shit itself for a few months it will be looking good going into any future election.