Yup, that's why there is a 75% threshold so your scenario won't happen. 75% is more than enough. It means on the worst case, 75% agreed to leave at most 25% behind. I doubt 25% will stick with the old chain anyway.
The other 25% which are almost 1/3 of the system are being forced to take action. If we end up being a system where the majority can push around the rest, then we are not better than fiat.
Then you don't understand how the actual fiat system works.
1/3. It equals 1/4.