75% is not meaningful. If a proportionate or more part of economy does not move the new coin will not have significant value and the miners who switched will be facing huge financial loss.
At that point they will turn off miners and both coins will have very long confirmation times.
They might switch back to the chain where most nodes & economic value reside in a desperate effort to again start making money and not go bankrupt.
Miners follow the Nodes and economy not the other way round for HFs. If Bitcoin miners tomorrow decide to mine DanielCoin with 2mb, DanielCoin will still be worthless and they will be forced to come back to Bitcoin with their tail between their legs. This is exactly the case with creating an BU altcoin.
The HF enthusiast were very confident before and were proven wrong by ethereum. They are confident again and are wrong again, but risk doing large damage to Bitcoin. In the meantime Coinbase users are still waiting for their eth classic. Something Coinbase laughed as impossible before eth HF.