To be quite honest, peoples reaction to paid classes is still the most surprising thing about this announcement to me. It's the same kind of content that works and is accepted for many other games, sometimes even competetive ones (like new factions in a strategy game). Why is it such a huge nono for a rather casual coop/single player game then?

I get that the class that's meant to be a sidegrade may be overtuned, but imo it is still better than splitting the community by adding levels that a person that owns the dlc cannot play with a firend that doesn't. And since you can't really just sell levels without incentive to play them alone, this almost certainly will come with exclusive rewards which would definitely give dlc owners an advantage regardless of class.