Aaannnd... we are in altcoin section! Even if it's valid bitcoin block, in main chain.

Activating a hardfork based on what miners do is really bad. You could easily have a situation where 75% of miners support XT but none of the big Bitcoin exchanges or businesses do. Then miners would start mining coins that they couldn't spend anywhere useful, and SPV users would find that they can't transact with the businesses they want to deal with. The currency would be split, and in this case XT would be in a far weaker position than Bitcoin.

The possibility of this sort of network/currency split is what makes XT not a "legitimate hardfork", but rather the programmed creation of an altcoin. A consensus hardfork can only go forward once it has been determined that it's nearly impossible for the Bitcoin economy to split in any significant way. Not every Bitcoin user on Earth has to agree, but enough that there won't be a noticeable split.