The problem if Momentum stood for election, is that they would split the Labour vote and pretty much ensure that they couldn't get elected. The Left is split too much already with the Greens and (part of) the Lib Dems.
Starmer needs to find a way of getting rid of them, without them standing against the party IMHO. Like Kinnock and Smith got rid of the hard left militant head bangers in the late 80's / early 90's.