Seat and Cathedral were added in later on, and Cathedral was massively nerfed, I'd take my bets same is gonna happen when Mechagon gets added to m+ rotation but that won't be until 8.3 or so. Blizzard tunes these to be a "challenge" on m0 when they're added and then with m+ scaling on top they get out of hand. Same thing with Karazhan 2.0, some abilities like Moroes garrotte had to be nerfed a lot to stop being ridiculous on m+.