Problem is, Momentum is associated with Jeremy Corbyn, and Jeremy is associated with trying to stop Brexit from the opposition benches (not even in power). But the idea of a similar grassroots movement isn't bad in itself. It has to speak for ordinary people or it becomes, like Momentum did, a fixture of an out-of-date establishment. Another thing about Momentum is it pulled in a lot of middle class people and students, whose views are likely to be PC and liberal, but the Labour Party needs its core vote, and its core vote is neither PC nor liberal. The class divide has never been wider, and the class knowledge (one class's understanding of another) is very poor at present. This is how Johnson was able to take the Labour vote over Brexit (unintentionally), and how Starmer and Corbyn have lost that vote. If we're talking Labour, a grassroots movement needs a full understanding of the core vote for it to have any more effect than simply choosing a new leader.