But then what happens if a lot of xt miners are also producing blocks to the point that the forked chain had enough halshpower to survive?
That's the point.
You can't REALLY know how much power is in each side BEFORE the fork, only AFTER the fork.
What will happen if we get that 75% of blocks were mined with those bits setted but, in reality, the hashpower is 49.3% and 50.7%. Goodbye Bitcoin.
This fork COULD BE a mess, maybe not, but could be in the scenario that I'm describing. This is REALLY necessary?, it is too much risk mate. We should stay with Bitcoin Core and make a new version of it, not building ANOTHER client to fork the blockchain.