"Dave paid the DM $50 so he gets the level 20 character. That's just the way it is, you can like it or leave."
Differences in player power will break cooperative games just as much as they'll break competitive games.
"Okay, so here's the kill stats. Dave: 1,422 kills. Sally: 3 kills. Bob: 4 kills. Lacey: 7 kills. Good job working together everyone!"
Someone getting advantages from pay-to-win would absolutely make people upset, myself among them.
You can be Nob, I'll be Aragorn.
Have fun tending the Prancing Pony while I'm off at Helm's Deep.
It's pretty obviously not a non-issue, no-one wants to be playing the Red Shirt/Peasant/Pikeman/Villager while their mate's running running around being a Jedi/Ranger/Paladin/Spock/Special Ops/Cyborg/etc. just because he dropped $100 extra on a game you both paid full retail for.
I was one of those twelve. First Mass Effect game I didn't finish, let alone play over and over again, because I realized I just didn't care about my party members.
If Battlefront hadn't made it pay to win, the gambling conversation wouldn't be happening yet.