Late joining the thread and catching up but I think something is being grotesquely missed here.
I am disappointed to hear that the demon princes are being powered down because it effects the MYTHOLOGY of the game. Say it with me - the MYTHOLOGY of the game.
D&D is in the first instance a game to be played and one can boot about this rule or that, this approach or that. It also presents a story line, a mythology. Most often this is seen in specific settings but divorced from settings (Planescape now being a dead setting like so many) The Great Wheel is its own mythology, always has been, and the lower planes are perhaps its richest source of myth.
When a powerful extra-planar entities stats are done over, particularly if inconsistently or with no in game explanation, the mythology is damaged. The hero/villain is suddenly less so and for no apparent reason. Canon is no longer as canonical.
For those who enjoy reading about and tracking the mythology of the game, depowering with no in game reason, is just bad design, even if it might be justified in terms of pure mechanics. D&D for many is not just the mechanics of the RAW. It is the mytholgy and the story. Its the read not the rules. So even if you never play the beastie and just read about it, depowering is an issue.