Paid relays are going to wind up leading to centralization and censorship. Once money is involved you gotta worry about lawsuits, censorship, control being in the hands of a very few. If you wind up with a monopoly or duopoly or where a few companies buy them up then what? Your right back at those relays choosing who can talk to who on their massive centralized server. Maybe they refuse to let you even use their service. Once they are big enough it won't be any different then blocking you from twitter. Sure, you can go somewhere else but what if everyone is using a relay that won't let you participate.