> all you need to change it is a majority of hashpower

Or rather the majority of actual users. Hard forks occur because people install and use the updated clients. If 90% of the miners decide to mine on the “bad” chain, but 90% of users switch to the “good” one instead, the “good” would likely still win out in terms of market cap and recognition (and the miners would naturally have to follow).

Because difficulty does not adapt dynamically, if the miners do not move then the fork will be defunct because it will take literally days to weeks to mine a block, and to mine the 216 blocks that would trigger a difficulty adjustment would also take proportionately longer. So transactions would sit in the mempool and the currency would be mostly useless.

Agree. Bitcoin is more of a social concept / weighted democracy than an algorithm. If the participants decide that a new / incompatible hashing algo is needed (e.g. post quantum), it will be forked and everyone will use that.

The odd part is, the governance model is not defined. At the end of the day, some person or group has to decide what the new "real" Bitcoin is and the average holder just has to go along with it.

I think for most bitcoin users, the main usability concern wrt forks is being on the most secure chain; i.e. hashpower. If you're willing to trade being on the most secure, most historic chain for specific technical features then you're probably on an altchain already. In practice every hard fork in history the chain with the most hashpower has retained the ticker, meanwhile the users are never organized enough to do anything but follow that decision.