Usage is not equal to viability. The problem I have with it is that it's overhyped which also shows in the usage. It doesn't deserve that much. In most cases Groudon doesn't pull off as much as people really think it does, or expect it to do. It isn't as versatile as people say it is. It only has about 3 sets that see use. And when you look at preview it's usually easily identifiable. There may be a gap between 1 and 2 in usage, but it's definitly smaller in terms of viability.
I do agree with you that usage does not equal viability, however, nomming Pdon from S+ to the top, middle, or bottom of S does not seem logical to me, like at all. I wasn't planning on responding at first, but since so many people seem to think it doesn't deserve S+, let me give you some facts.
Its stats are monstrous and it's ability is fantastic
It has the 4th highest basestat of all pokemon, and one of the ones above it is Mega Rayquaza. On top of this it has an ability that, with the amazing typing it has, completely neutralizes its x4 weakness, leaving only ground to hit it hard with.
It's the most versatile pokemon in ubers
For some reason most of you seem to think of one specific set when considering this drop (at least that's what it looks like to me). Rock Polish mixed/physical/special attacker, Spdef defensive rocker, 4 attacks balance breaker, Double dance, Para dance, Defensive Rest, etc. etc. It has so many different options for moves, spreads, and sets that no other is its equal in terms of versatility.
It keeps monsters in check
You can argue all you want about how well it does it, but the fact of the matter is that
it does keep pokemon like Primal Kyogre, Xerneas, and Ho-oh in check.
It's extremely splashable
No matter what 5 pokemon you have on your team, you can pretty much throw a Pdon on it and it will make it better. Stall, Semi-stall, Balance, Bo, or Ho, it does not matter.
I might have even forgotten to mention some reasonings, but that doesn't matter. The fact still remains that it is the best pokemon this tier has to offer, without equal. I think it should stay in S+. And tbh, I don't see why you'd nom it in the first place if it is only to move it down from S+, while it still stands above the rest of the pokemon. But again, leaving that aside, it is the best pokemon this tier has to offer, and sure, it's usage is no proof of its viability, but the reasoning behind its usage
does prove why it is S+.