The problem with outright nerfing all the megas is that the vast majority of them aren't overpowered. The Mewtwos, Lati@s, Hoenn trio, Diancie-M, Gengar-M and, to a much less extent now, Megabro, Audino-M, Megadactyl, mTtar, Megados, and Maggron are the only good ones. By nerfing all megas, we needlessly hurt the megas of Venusaur, Blastoise, both Charizard, Alakazam, Kangaskhan, Pinsir, Ampharos, Scizor, Heracross, Houndoom, Blaziken, Gardevoir, Mawile, Medicham, Manectric, Banette, Absol, Garchomp, Lucario, Abomasnow, Beedril, Pidgeot, Steelix, Sceptile, Swampert, Sableye, Sharpedo, Camerupt, Altaria, Glalie, Salamence, Metagross, Lopunny, and Gallade.
Aka, it'd nerf 9 very good pokemon, 6 viable and good but not certain not broken Pokemon, and 35 Pokemon who are currently outclassed or are already just lacking. In other words, we're nerfing 41 Pokemon, more likely 44 Pokemon since not even all of the top 9 are broken, to nerf the big 5. Unless game mechanics changed that made using megas without their stones impossible, I can't get behind nerfing all of the megas just because we don't need to nerf the vast majority of them do deal with a few problems ones.
Plus one of Pdon's best sets is Desolate Lands anyway, so it really doesn't hurt it that much. All the nerf would do is force it to variants of one of its best sets and force it to be immune to Trick while about an estimated 40 or so Pokemon become completely unviable.
Edit: As for a suspect thing, we could hold a suspect Round Robin tournament or something of the like.