By the simple fact that isn't compatible with a URSF. In this scenario, the miners create a Bitcoin client to reject a UASF which causes a hard fork.

When a UASF node runner attempts to do something sidechain related in that version of the chain, everyone can see W^+1 for that spend condition.

I suspect most miners will select for the "do nothing" action when mining a block rather than the W^ action. Which means only a group of interested miners are influencing this side chain you're apart of. There will be many (if not most) chains in which small interested mining groups attack sidechains in the UASF chain to encourage users to run a URSF client and get access to their funds.

This is not a 51% attack and it does not mean 51% of Bitcoins hash power is being used to attack Bitcoin