Reasoning: A high-quality community will probably grow over time as new users discover it. As it grows, the quality of its user base will inevitably regress to the mean (i.e. decline).

It's more like the quality of posts will regress when the high quality community has largely shot it's wad, and is satisfied with their answers. When newbies come, they'll get pooh poohed by their elders, and often be intimidated. A newbie discussion starts, but an old timer will chime in with the "solution" and talk will peter out.

The culture of peers figuring it out dies, to be replaced by cranky old smarty pants who squash the kind of flailing about they used to do as "useless noise".

Having "Joined in Year XXXX" classes, each with their own forum, would be a way to segregate freshly minted eager beavers from cranky old codgers who have heard it all a thousand times before. Everybody wins. A class of Johnny Come Latelies might even come up with new ideas to challenge the geezer consensus.