Well, what matters is if the exchange is profitable, and how low the volume would have to be for the exchange not to be profitable.

B/C as long as the exchange is profitable, why would they shut down?

I don't know the economics of running an exchange. Did Poloniex use to be very large and has been shrinking? If they aren't decreasing in volume and can provide a market for Monero that people trust (read: coins are safe), then it will continue to exist and fulfill a niche. The 2 reasons to switch are trust and liquidity, and right now Poloniex has both.

Poloniex is betting on a certain horse to succeed (due to volume, probably didn't have a choice). If XMR does succeed, Poloniex will be doing quite well assuming they are indeed taking the XMR fees from the trades and hodling.

Saying it is irrelevant is only proportional. I use it and love it, so it is not irrelevant to me.  However, if the you believe that the exchange is losing money and that is increasing the chance of a cut-and-run, I'm all ears.