If they are expelled then they aren't a state anymore, are they? The Constitution does not apply and they aren't entitled to representation. So show me where the Constitution forbids that.
You have a short term memory problem, don't you? We just went over this uptread. Any action to expel a state violates Article V because that act simultaneously deprives the state of representation and thus violates the Constitution.
I suspect that membership in the Senate was protected in the Constitution because the Senate was an expression of state sovereignty. As John Lansing of New York said in the ratification debates:
I believe it was undoubtedly the intention of the framers of this Constitution to make the lower house the proper, peculiar representative of the interests of the people; the Senate, of the sovereignty of the states.