It's not, but you hit the interesting point that we're already at a number of public holidays where employers don't automatically grant all the federal holidays

It's not, but you hit the interesting point that we're already at a number of public holidays where employers don't automatically grant all the federal holidays. (I have seen employers make holidays "floaters" and let people choose which floaters they want to take off. Implicit is that they can't take all the floaters without using vacation days.) So adding another 10-20 would likely not change actual PTO all that much.