I don't see why I have to keep repeating myself, but you're thinking about it the
wrong way. Any one who uses Sheer Cold intelligently will not try to rely on a 30% chance to attempt to kill a DD Rayquaza, that is not a reliable method. You use it on DEFENSIVE approaches, it's simple risk/reward. In some situations the reward of Sheer Cold far outweighs the risk, even if it misses the first two times - spamming it vs Gastrodon, what's it going to do back? Even Ferrothorn will not want to risk setting up hazards.
What you are saying shows an overall poor understanding of the greater depth of use that Sheer Cold has, why risk what you don't have to? If your team stupidly relies on Sheer Cold to beat DD Rayquaza or any other set up sweeper you deserve to lose, or only win 30% of your games. Reliability in team building is actually hugely important, if you want to rely on such a variable chance to workaround your Kyogre then by all means do so, things even out in the end. It's just another strategy.
Coin flips will not determine a significant amount of battles unless the players both rely on using a large amount of luck-based strategies. If both teams face off using 6 OHKO move users then yes, it's a crap-shoot. If one player using Hyper Offense vs a team of OHKO users than Hyper Offense
will win almost every time.
OHKO is just another element of chance, and a legitimate strategy, as is using Darkrai, CM Ghost Arceus or simply anything that can win or lose based on a chance. Such is Pokemon.
Also Ubers is by no means endless set up sweepers, stall still exists, defensive play still exists, rain balance is still the most consistent style of play. You'd be stupid to argue otherwise.
Poppy how would the meta handle RestTalk Kyogre with Sheer Cold and Scald? Stall and balanced teams are fucked up by it, as Kyogre gets many switch in oportunities, has reliable healing, and the opponent does not have many dangerous set-up sweepers.
Against offensive teams, Kyogre can still find time to use Sheer Cold. Don't forget that physically defensive Kyogre walls/checks many pokes, such as Leftovers Ho-Oh, Dialga, Kyurem-W, Darkrai, Terrakion and some else, so he will still have his chances to abuse Sheer Cold. It's not as Scald is something to laugh at either, but the fact that Sheer Cold will butcher balanced and stall teams make it seem uncompetitive.
As always, it's impossible to say for sure what will happen, but Sheer Cold Kyogre isn't unstoppable by any means, a lot of attacks CAN 2HKO it, and Rest is not reliable recovery. Certain workarounds can be done, Substitute Pressure Lugia beats this Kyogre 100% of the time if its Substitutes are EV'd to survive a Scald (as does Substitute Giratina). What we know to be the current conventional stall / defensive play would struggle with this set immensely, but adaptation is sure to happen, as it always does.
If your team is not prepared for Sheer Cold Kyogre then you deserve to lose to it, same with any other threat. At this point it does not seem that overwhelming preparations that leave you absurdly weak to anything that can be paired with Sheer Cold Kyogre (although who is to say there will not be when we test it?) for it to be considered a guaranteed, or even close to, way to win.