I disagree, I think boarding first for F is more important because everyone is grouped in a few rows and sometimes the first row bins are not usable. It is not uncommon for a late arriving F customer or upgrade to have no bin space unless the FA specifically saved a spot.
I've even run into the issue in J on some aircraft.

Y boarding is more spread out so you can always make sure your bag goes above or forward of your seat.

As for a lounge access benefit, it is ofcourse possible to give more people access to polaris or UC. UA would need to build more lounges to prevent overcrowding. That already happens with other *G at other *A airlines.
This will ofcourse never happen with UA.