Yeah I played Cata. Remember this guy called DEATHWING?"Have you played any Cataclysm? You know, when the world blows up? Because of the Old Gods" - that was Blizzard's answer, word by word, to a guy who asked them to address the same issue you brought up (why we haven't seen any catastrophic event even with two down). It wasn't even something new and have been linked over and over many times, so I'm not sure how you've missed it.
I don't know how bad I'm at Warcraft lore, but I'm pretty sure that at least in this regards, I'm way ahead of the C'Thun & Yogg are still allliivveee crowd. Especially given that we have confirmations both in-game and out of it.
(1) Omniscient Narrator in the in-game quest text flat out stated that C'Thun was destroyed and dead.
(2) The comic stated the same as well. And on that note, Blizzard - in particular, Afrasiabi - clarified very recently in Legion that they only meant "Med'an being the Guardian" is not canon, but the comics are still canon.
(3) Blizzard Q&A section answered that C'Thun and Yogg's deaths caused the Cataclysm. Check back a few pages in this thread, or pretty much every single Old God related thread in the last 4-5 years.
So if you are saying C'Thun and Yogg are still alive as of the lore right now, you are relying on the word of in-game Old God follower NPC over the omniscient narrator, the comic and WoG itself. Next you just need to call Blizzard developers liars and you will fit nicely into the category of "shouldn't-be-bothered-with" group. Have you ever asked yourself why have the "C'Thun-and-Yogg-are-alive" group never been able to provide any blue post stating such?