You have to take into account cards though. Since Mega Gyarados is water, it can take advantage of Archie's and Keldeo. In Standard, there's Mega Turbo.
Tyrantrum EX focuses on OHKOs and it can get setup in 2 turns with DDE. Primal Groudon amazingly got resurrected during Regionals and it focuses on OHKOs. Yeah it's slow but Omega Barrier has great defense so that doesn't matter. M Ray focuses on OHKOs and can beat NM, Keldeo and Magnezone/Raikou also focus on OHKOs, both Mega MewTwos focus on that as well. Need I go on? The point is, while OHKO decks always existed, this format focuses on it. Saying they always existed is shifting the discussion. Weren't you replying to someone saying that this format isn't a OHKO one?