You are correct.  The only way 50+% of miners can override non mining nodes is if there aren't any miners left on the original ruleset to process transactions.  However, they could drastically slow things down until the difficulty readjusted with 50% leaving.  If enough miners remained with the original ruleset, we would then have two blockchains and everybody with coins before the fork could use those coins twice: once on each new chain.

Edit:
Although if you received the coins on one chain, you could rebroadcast the transaction on the other chain and take those coins too since you have the private key for the destination addres and the transaction is properly signed.