You would essentially "vote" with which fork you were choosing to acknowledge with your own node, should you be running one.

Also I'm fairly certain a majority of hashpower, not just users would need to be on board with the change.

You could argue that as the primary beneficiaries of such a change (increase cap => increased distribution to miners) they would be for it, but I think most rational actors see the catch 22 of trying to profit by removing one of the core attributes that makes bitcoin valuable (it's scarcity)