so why aren't we even contemplating expelling a terrorist state from their seat at the top table?

UNSC permanent seats aren't gold stars for being nice or given out because we like a state.

They're there because that state is a major military power with whom outright war to impose the will of the others on is generally considered to be unthinkable. Instead of a UNSC mandated action running into World War 3 with them, we get a veto.

Russia has the largest nuclear arsenal in the world.

It is possible that technological advancements will produce a counter to that that renders it impotent, but until then, nobody is going to kick Russia off the UNSC.