This is another issue I really have with Keldeo is that its basically hard stopped depending on what set its running. Specs is hard to switch into, but relatively easy for offense to force out because of its lower speed tier, not to mention even the specs set does piss if it gets locked into a bad coverage move. SubCM can be a decent Stallbreaker, but its lack of initial power means it has trouble finding set up chances until anything that can beat most bulky water types is eliminated. No matter what Keldeo runs, something is going to give it severe problems. Compared to the other S-Ranks I find Keldeo is neither proficient enough at one role nor effective enough in a diverse number of sets to feel S-Rank to me. Compared to other S-Ranks:
Metagross: Has a better speed tier, and only one or two sets, but the sheer power behind its moves thanks to Tough Claws and its relatively minimal need for coverage means Metagross has everything it needs to be a threatening wallbreaker and/or Agility Sweeper.
Landorus: This thing is always a frightening wallbreaker, the choice of its 4th move just defines whether it wants to give even more hell to Stall, Offense, or balance.
Altaria: Altaria can either be an offensive sweeper, a defensive win condition, a Wallbreaker, or even a decent cleric. Altaria only performs maybe at an A+ level on any set, but the sets function so differently but effectively that you have to prepare for Altaria to do any of the above.
Clefable: I personally felt Clefable was a weak S-Rank as well, but I still would say it's closer to that line than Keldeo. Clefable can just do a lot of different things as well, ranging from defensive utility to LO lure/offensive sets, while being a decent defensive glue for balance teams. While not nearly as varied as Altaria, Clefable can do a number of things very well for a team, especially considering it isn't a Mega nor does it cost anything like another form or something.
Keldeo's choice of set has very little effect on what it typically takes to answer him. Yes, maybe there's some mons that can't answer the SubCM set as well as the Choiced sets, but the answers to to be the same: Talonflame, Bulky Waters, fast attackers (though Scarfers usually for the Scarf set in particular). Keldeo is never something I find myself weak to because the sheer number of checks he has means that taking the Metagross or Landorus approach of trying to wear out his own checks/counters doesn't work as well because Keldeo is much more easily pressured by them.
The win percentage argument can't be discounted just based on that because Keldeo has a significant usage rate, compared to things like Pikachu Belle appearing... once during the tours? If Keldeo is used on, say, 60 teams and only has a 40% win ratio, that would have to mean something considering this is an S-Rank mon that is supposed to be able to pressure even teams that do prepare for it. Granted it shouldn't dictate the ranking alone, but with something as significant as S-Rank Keldeo, it has to indicate something about how it fits into Tournament level play.
Tornadus-T might not be metagame defining on the level of the S-Rank mons, but it is a significantly useful mon on the rise that is a consistent answer to Keldeo. And as I noted earlier, there are at least 20+ Pokemon that can answer Keldeo to reasonable effect in the A- and up ranks. None of those mons are defining the meta on the S-Rank level, but if the S-Ranks define a meta in which these are doing well, their presence still hurts Keldeo. The argument could be made that these mons are also rising in response to Keldeo, but at least half of these Pokemon were in the upper ranks already, and quite a few others (like Serperior) didn't tend to take Keldeo in particular into consideration in moving up. Aside from maybe one or two like Celebi, I don't feel like any of the high ranked Keldeo answers got there in SIGNIFICANT effect because they answered Keldeo. You mention Starmie moving up didn't cause Keldeo to drop, but you also noted by the same token that Torn-T, arguably a more influential mon, rising shouldn't justify dropping Keldeo.
Still, as I noted, Torn-T and Starmie are among mons that have been rising that check Keldeo, but I doubt anyone would say it's BECAUSE they beat Keldeo the same way Mandibuzz rose because she answered Aegislash well. I do NOT claim anything to have that same level of pull, but Metagross for example was a notable factor in Slowking being nominated to rise, or how countering Landorus is a big part of Cresselia's ranking for a while. I have never seen "it checks Keldeo" listed as a significant argument to raise anything in a nomination the way other S-Ranks have gotten.
Even with its S-Rank now, Keldeo is never something I find myself particularly weak to, to the point of outright changing out members or reshuffling cores. At most I find myself altering a couple coverage moves and that's it. I don't consider Keldeo meta defining in the sense that the meta follows him, but rather that Keldeo has a slight degree of adaptability (in spite of his shallow movepool) and one role that is tricky to invalidate or weaken in the Meta because of Scald, regardless of what I think of the 30% "hax or just muscling past" arguments. That said, Keldeo does that role pretty well, but not well enough that you have to bend entire teamslots to it specifically.