One important dynamic I think people often miss is that party discipline is weak in the US. Schumer can’t make the caucus go along with a shutdown fight; he has to convince them it’s a good idea, and be confident they’ll stay convinced for long enough to get what he wants, because otherwise the 7 most moderate members will happily defect and start writing their campaign ads about how they’re independent minded who won’t be pushed around by anyone. He’s already got 3 defectors in the current fight.