I dont see any arrogance to be honest. I saw loads in momentum that they were right and "capitalism will fail". I see the front bench now as a government in waiting. Potentially.
I'm not sure I've ever seen or heard anyone in Momentum say that. Either way, do you not think, the idea as espoused by staff that if people they like don't run the party they can throw their toys out the pram and wreck things, as a fairly arrogant and entitled way to behave?
If that culture is allowed to continue, Labour will not win. As for government in waiting, I don't see it at all. Who are the stand outs in this cabinet? In the latest poll Labour have slink to 20 odd points behind the Conservatives. We look a million miles away from government. Yet we were sold on the idea that just changing Corbyn would lead to this great renaissance and huge poll leads against (as you put it) "the worst Tory party in history" (it's not, the one that Blair faced was).
The truth is this goes much deeper. The rot goes much deeper. There's no magic bullet here, and trying to act like a government in waiting will be massively damaging (as it was for Corbyn when he took advice from people inside Momentum to start pursuing this policy).