As noted, this could hurt WPs a lot.

For the rest of us mere mortals, it introduces an option that we can enjoy on the other domestic carriers already (but, not for domestic, only for international flights).

The theory must be that the additional revenue from low-class flyers paying will exceed any loss of revenue from WPs leaving.

On international long-haul, perhaps the theory is that WPs will still have the best access to upgrades (to the expanding Y+ offerings, as well as to J) and so perhaps the hit will be more gentle?