If non-US carriers start handing out UG to people who don't pay for them, they will wind up with premium cabins like UA as the result of UDU. Who would pay for J if they can get it for discounted Y? Far better to let J go out empty with every seat in Y full than to hand out the J seats to status pax in Y. (Who gets the OPUP if Y is oversold is a different question).

I would much prefer the US domestic carriers to follow the non-US model. Make the premium cabib (F, BE or whatever) a real premium hard and soft product and then sell it. I'd pay.