Remember, long ago people referred to this country as "these United States" and not "the United States." It is a willing association of states based on common protection and interest.
So would you agree that the other sovereign entities can expel a state against its will?
It is a willing association of states based on common protection and interest.
And bound by the same Constitution that protects all of them equally. So if a state walks out of the Union and repudiates responsibility for debts and obligations, and takes with it every bit of community property it can get its hands on without compensation, then what are the other states supposed to do? Sue? Where are the protections for the remaining states under circumstances like that?