If the consensus is still "well chaos can just soup daemons and other Legions to stay competitive", then what's the fear of giving them a mono bonus? Can't have both, no power gaming there. It at least would give us a choice and let those that want to play an actual legion play the darn legion.
I think the reasoning behind not giving Chaos a mono-faction bonus is because we have a mechanic that can directly circumvent it: Daemonic Ritual.
What's to stop a player from building a 2000 point list that's 1500 points of a single legion of marines with 500 reinforcement points, recieving their mono-faction bonus, and then summoning in 500 points of Daemons mid-game?
While there can be an argument made for mono-god dedicated armies getting a bonus, the Undivided armies like Word Bearers or Black Legion could bring in any flavor of Daemon they wanted. I really doubt that any chaos army will get a mono-faction bonus because of this.