We are in the scenario where all pools agree upon a protocol, and the full nodes want another one. Pools only care about their block being accepted by other miners
nope. because if pools right now made blocks 465623-465723 that were, say using MD5 hashing..
by this evening.. they would find out that all the merchants wont see their rewards of 465622
the pools would have most definetly realised that merchants wont buy their 465622 reward by at most 465623
Well, they would realize that all merchants are simply locked out of all transactions, unless merchants use the new rule software on their full node, or connect their wallet to a miner node. Because there aren't any other transactions in accepted blocks.
So yes, miners can't cash out their coins to users, as long as ALL USERS decide not to upgrade, and accept being locked out of bitcoin, their funds and their transactions at the same time. The question is why users would keep themselves locked out, while they can be up and running with bitcoin by simply running nodes that accept the existing block chain, or by connecting their wallets to such nodes, of which the miner pool nodes are of course examples.