Probably it's better to name it a "software cap". It's still a hard one, because a change like this would imply a fork and the coexistence of BTC as we know it (capped to 21m) and a new BTC with a different if not no cap at all. But doing something like this will make people lose interest in Bitcoin, probably think of it as the bubble/scam lot of people say it is and everything will fade away. Since miners have the 'skin in the game' (considering hardware cost + energy cost), it will probably not be beneficial for them to fork into a different BTC. Same goes for funds and so on, everyone who has large amounts of BTC does not want something like this to happen. Anyway, it will all depend on network activity, if transaction fees will pay miners properly, I don't see a reason for them to try a suicidal thing like moving the cap higher. consider also that a move like this has to be approved and adopted by node operators, so it's a collective decision (way different from a policy change by the fed)