I really don't think that's the game that Momentum is playing.

Keeping Corbyn and the Corbyn cabal in place will keep the mozzies, snowflakes, and assorted Jew haters "on song" while a last minute swap with someone apparently less of an askhoul would attract more of the Labour old guard back to the fold than would be lost from the filth contingent.

Never underestimate Momentum. Their name is especially well chosen considering what can be gleaned by what they're doing.