They key point is that the owner of something other than bitcoin is actually the person who determines what bitcoin is (when they accept bitcoin as payment). Just like when you go into a shop to buy something the shopkeeper decides if your cash is real or counterfeit. Extending this out to the system as a whole, the rolling set of participants trading something in exchange for bitcoin have 100% of the hard power over the bitcoin protocol (not devs, miners, node operators). Of course some of these people might also be devs or miners or node operators, but that isn't what gives them a vote.
The sender of a transaction is paying miners for immutability as a service, and they are doing this on behalf of whoever they are trading with (as they are the party that determines if the transaction is valid and immutable, not the sender).
In 2017 most of these trades were already happening on exchanges just as today, but there's more than one exchange and a quick way to lose all your customers is to be on the wrong side of a fork, so what they are really doing is validating on behalf of their users (and users tend to withdraw and self custody at some point, validating against their own node or SPV against public nodes).
