I've mentioned this elsewhere, so I'll c/p some chunks of my arguments there re: Krookodile, Entei and others.
Most of the arguments I've seen for Krookodile rising focus on how easy it is to fit Krookodile on teams, how it puts in work versus most matchups. I've got no argument there. Yes, there's practically no drawback to putting Krook on a team, and it fits on a myriad of playstyles. The same is true about Sylveon, which is probably the single most splashable Pokemon in the tier right now, and almost every argument I've seen for Krook being S-rank would apply to Sylveon as well. To be honest, I think Krook, Entei, Sylveon and (sorta) Conkeldurr are all really hovering in an odd space right now - they're better than the rest of A+, but not as good as the mainstays of S (in particular Hydreigon, Celebi and Suicune - I'll get to MAero in a bit).
Regardless of how good Krook is, I don't think it's at all quite at those levels. The issue to me is not that Krook & Co. are all suddenly S-rank, but that there's a growing gap in the A+/lower S ranks. If we're keeping the ranks as they are, then we're in this sort of odd position where either Krook, Entei and Sylveon stay in the same rank as Mega-Beedrill and Gyarados (which feels wrong), or we move them up and put them in the same rank as Hydra and Celebi (also feels wrong), or we shift the rankings downward and move stuff like Mega-Bee down (which will make many people think we're saying that they got worse, when really it's that the others got better).
But anyhow, while things like Krook and Sylveon are incredibly good at getting their jobs done, and are really crazy splashable, they just plain don't do what Hydreigon/Celebi/Suicune do, which is shape the goddamned metagame around themselves. Hydreigon is just about the hardest Pokemon to switch into in the game, offensively checks a major portion of the tier and more or less singlehandedly forces teams to run a Fairy or trade with Hydreigon every time it comes in. Celebi is the single most dangerous setup sweeper in the tier, has completely changed how every team operates right now, and has the tools to beat just about every one of its "counters." Suicune is rivaled only by Sylveon in the amount of crap it shuts down, while ALSO being probably the second most threatening setup sweeper after Celebi.
As for Mega-Aero... I never thought I'd say this, but Mega-Aerodactyl is kind of suffering right now. It's funny because I still use it more than any other Mega, and it's still crazy good, so it's really hard for me to say that it's getting worse. If anything, you'd think that the rise of things like Celebi would really help it. Pursuit is still a crazy-busted move (let's be real, Pursuit is the only reason anyone should ever be considering moving Krook to S rank), it still checks a ton of shit. So what's the problem? Well, the issue is twofold. First, the kind of HO teams that Mega-Aero particularly excelled against have really started to dwindle in favor of BO teams that are way less threatened by it. Second, there are things that Mega-Aero should check, but it just doesn't. In particular I'm talking about Celebi and Conkeldurr, which take anything Aerodactyl can throw at them when healthy. The fact that they're both popular SHOULD be a selling point for Aero right now, but in practice you need to stack redundant checks to those two because if either of them are healthy, Aerodactyl doesn't actually beat them. It means that Aerodactyl is no longer the emergency button it once was, and often I find I have to sack something else to get chip damage off on Celebi/Conk before I can bring in Aero to revenge. It's still really really amazing, better than anything in A+, but I don't think it's quite at the level of Celebi/Hydra/Suicune.
Speaking of Suicune, here we have the opposite issue. By any reasonable metric, the rise of Conk and Celebi SHOULD mean Suicune drops, but in practice, I've actually found Suicune if anything better than it used to be. Part of this is because unlike Aero, Suicune was actually able to adjust to these drops so that neither one necessarily checkmates it. In particular, more offensive Suicunes have been growing more popular, especially SubCM (including variants such as Vincune, Ice Beam Cune and SubCM Roar), which don't necessarily counter Celebi/Conk but at least mean that Suicune isn't setup bait. Also, teams have been slapping on a Celebi or a Conkeldurr and assuming that they're covered against Suicune, which is something that a well-built team can really take advantage of. As I said above, I think Suicune is the second most dangerous setup sweeper in the tier, and I feel like I've won more games lately with Suicune than with any other Pokemon. It helps that Suicune never really was expected to check shit like offensive Grass types to begin with, so it doesn't actually strain teambuilding to account for Celebi when you're building with Suicune.
Anyhow, that was a really long and convoluted way of saying that I think the rankings look something like this in my head:
BEST OF THE BEST: Celebi, Hydreigon, Suicune
ALMOST THE BEST OF THE BEST: Aerodactyl-Mega
REALLY GODDAMNED GOOD: Sylveon, Krookodile, Entei, Conkeldurr
VERY GOOD: Everything else in A+/A