Good question!

So the exchange rate let's say is 1 eCash for 100 Zside for example

This unit conversion is unnecessary, Drivechains are supposed to be pegged 1:1, adding a multiple on top just makes it harder to reason about.

So the pool is drained, a, b and c go to remove their Zside and their attempts are refused because the pool is empty. Is that a legitimate concern?

Yes. This legitimate concern of "turnstyle" withdrawals extends to any bridged coin, even if it doesn't have extra privacy.