Think of it this way. There is no "official" bitcoin anything. If 80% of the hashrate adopts a fork and 20% of the hashrate sticks with the old protocol, does the side with the 80% become the "official" bitcoin? I don't know. If I haven't "upgraded" and blocks are still being mined, ... then as far as I'm concerned, I'm still on the same bitcoin that I've always been on. It's possible the miners who were on the side with 80% of the hashrate will eventually abandon that side and return to the original protocol.

That's why "crypto-economics" is not just hashrate, and not just a protocol, but something more complicated. It involves game theory, economics, and cryptography.

There has even been an unintentional hard fork:

Has Bitcoin Ever Hard Forked? https://blog.lopp.net/has-bitcoin-ever-hard-forked/