I already explained how people think Gyarados is a check, hence why people will be switching in. Regardless of whether or not you think it should be switching in, people still do switch it in. Also you said Gyarados hard counters this core, but now you're saying Gyarados has to wait until something dies to switch in.
Just because a couple of morons think Sub DD Gyarados can counter Specs Keldeo doesn't mean its a good move bringing in Gyarados (for example). If you can prove that at a high level, players are constantly switching in Sub DD Gyarados into Specs Keldeo, with SR up, to attempt to check / counter then ok, ill believe you. Scarf Keldeo is potentially a difference story since you can come in after a KO and have a realistic chance at setting up and sweeping but you were the one that posted Specs Keldeo calcs so I guess people are deliberately switching in Gyarados on Specs Keldeo to counter it???
Also again, read my post. I would argue that Rest Talk / Chesto Rest Gyarados (rare as fuck tho) is a good counter to the core, because iv actually come across very talented players that have actually used this core against me, and its exceptionally frustrating. Don't chuck in Specs Keldeo into the mix (when both in my experience and from this thread, id argue that its generally Scarf / Expert Belt Keldeo im seeing with this core). Yes, if Landorus-I spams U-Turn and I keep bringing in Gyarados if SR is up then yea, Gyarados will eventually die, Assuming SS is up, and that I never found a away to get SR off, and that SS is up, then yea, ill eventually die after a few U-Turns. I just want to correct a few things.
-Firstly, your 3 man core, has now become a 4 man core, with CB or Scarf Tar / Ebelt or Scarf Keldeo / U-Turn Landorus-I / Gengar or Jellicent. This is facing my 2 pokemon core of Gyarados + a spinner (say Forretress with SR). Thats all "my" core needs to effectively win against Keldeo + Landorus-I, while your core still needs a SRer somewhere (pushing it to 5) otherwise Gyarados can keep switching in. You understand, that's a significant amount of investment your team has just made to block spin, especially when Tar / Keldeo / Landorus-I cores don't have a lot of. ANyway I hate making this point, but in this case its a fair one.
-Secondly those U-Turns are not doing 9-10% to a Rest Talk Gyarados (you forgot Intimidate) so its more of a realistic 5-6% (ie equal to or less than my leftovers regeneration). U-Turn will do more if I move EVs into Special Defence from Defence, but im 2 lazy to make an arbitrary EV spread with a special defence bias. This means I can switch in on Landorus-I more easily, and that you really need Sand on your team, and up, otherwise that Landorus-I is going to find it harder to break me.
-Thirdly, assuming I lack a secondary Landorus-I check, or that I fail to predict those U-Turns, and that I never spin away SR, and that sand is up then yes, Landorus-I wins. In the process, Gyarados has switched in 3 times, and assuming SR is up on your side of the field... then your Landorus-I has lost 67.5% of its health (putting it on 32.5%) but because its used U-Turn to kill Gyarados, then its actually on 20% when it next comes in. You cannot tell me stripping off 80% from Landorus-I is anything less than effective, even assuming extremely favourable conditions from your Point of View, especially considering iv also prevented you from bringing in Keldeo + any other mon my Gyarados can try and get a free Rest on. As well as considering my 2 mon core...
Well I'm talking about the U-turn set here, which I believe to be the most devastating, so you can only really switch into HP ice...
Try HP Ice, Earth Power, Focus Blast and U-Turn (although id agree U-Turn forces a negative matchup for Gyarados, it can still switch in on it). If you use anyone of those other 3 moves and I predict it, then I get a free Rest.
Lastly, I want to make one quick note (since you missed it like twice...) when I said Gyarados countered the core, I was referring to Rest Talk Gyarados as the counter. The pokemon that "checks" the core is Sub DD Gyarados, since, even tho it is a risky switch in on Keldeo and maybe Landorus-I, if it gets in safely, it can push for a DD and sweep. It is the very definition of a CHECK not a counter. Don't bring Specs Keldeo into this since a) its rare in my experience) and b) it skews everything, and yea, if I was running a defensive team, id have another 1-2 Keldeo checks / counters regardless so its pointless bringing that up over and over again. Point being Sub DD Gyarados really shouldn't be switching into Keldeo unless it has a dam good reason like SR isn't up, or its predicting a SS / HP Ice (and that its prolly not Specs which is rare as fuck and you wouldn't be switching into it anyway).
Worth noting that I still think Landorus-I is uber. Sure, Rest Talk Gyarados might counter the core, but its not a great counter, and it still doesn't shift my perception that Landorus-I restricts my teambuilding far too much in order to deal with it. Agreeing with BKC on this one.