Of course they can if they configure their wallet to connect directly to a miner pool node ! And do you think that a merchant or an exchange is going to stop its business for a day, while the competition accepts the new rules on the chain, and gets its node and wallet up and running again ?
your ignoring the past and creating scenario's that never happened..
pools learn in seconds if their block is going to be accepted.. they wont just continue for 16 hours HOPING merchants adapt..
pools learned this lesson the hard way years ago..
it seems you are trying too hard to downplay satoshi's consensus mechanism and trying too hard to make bitcoin sound like the fiat system.
its time you stop looking at the end result and thinking "its just a database" and instead actually look at all the different security mechanisms that make bitcoin completely different to fiat.