When Classic will get activated, I doubt anyone will stay on the 25% chain. That chain will end up being 0.
You seem to miss the point - it won't be such a clear cut case and the exchanges *will shut down* (they would be stupid to not do so).
So the mining pools will end up with no-one to sell their BTC to.
Do you really think they'll keep on mining?
Why on earth exchanges will stop trading? Most (if not all) of them support Classic btw.