I can see some issues with your reasoning. Before I get started, it would be more convenient if you specified at the beginning which type you want it in S rank for, because the whole first paragraph of your post is left up to interpretation, which at times leads to misunderstanding.Just a quick nomination:
Celesteela: A ----> S
I don't honestly know why this thing isnt S ranked yet. For starters, this monster puts tremendous pressure on almost any ground build. ive been on the recieving end of this thing and no matter what set it uses its a massive pain in my backside. assault vest especially being the worst. Can't set gravity without taking an hp ice and losing landorus, and no other mon can really force it out 1v1 unless gravity is up or unless i specifically cteam this one mon. which just limits which sets i can use on what.
This thing also helps in the steel vs steel matchups as it can carry flamethrower/fire blast and/or Earthquake. which can force out every mon on steel if heatrans balloon is popped.
It just in general helps steel out by being a special mon to complement skarmory/heatran as well as being capable of being more offensive as to be a great ground immunity for HO steels.
It eats hits from things that normally trouble steel like, greninja, landorus i and landorus t, and hydreigon and is able to deal either massive damage, or just ohko it outright. Id highly recommend a jump for this thing
sorry if this is sloppy, its a bit rushed but i just had to get my thoughts out.
Regarding the Ground matchup, it's definitely true that Ground teams do get pressured often by Celesteela sets of just about any variant. However, Ground teams still have their fair share of effective countermeasures in Mega Garchomp, possibly Firium Z Garchomp on SD sets, Choice Band Mamoswine, all of which while unable to OHKO are still able to wear it down effectively:
252+ Atk Choice Band Mamoswine Icicle Crash vs. 252 HP / 4 Def Celesteela: 222-262 (55.7 - 65.8%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Sand Force Garchomp-Mega Stone Edge vs. 252 HP / 4 Def Celesteela in Sand: 169-199 (42.4 - 50%) -- guaranteed 3HKO after Leftovers recovery
252 Atk Garchomp Inferno Overdrive (120 BP) vs. 252 HP / 4 Def Celesteela: 254-300 (63.8 - 75.3%) -- guaranteed 2HKO after Leftovers recovery(at +2 it guarantees a OHKO)
These Pokemon are also able to place pressure on any potential switch ins, with Mega Garchomp in sand having no safe switch ins, and nothing on Steel openly wanting to switch into a potential Choice Banded Superpower from Mamoswine(Skarmory has a chance of being 2HKOed after Stealth Rock if Mamoswine chooses to Icicle Crash). But I do suppose that Heatran could make Garchomp use up its Z-Move. Even with that being said, these are still decent ways for Ground to deal with Celesteela outside of Gravity, and I wouldn't call this limiting at all. In a potential Steel v Steel matchup, Celesteela, while having no particular switch ins, will still be trapped and removed by Specs Magnezone a lot of the time after Stealth Rock to Assault Vested sets:
252+ SpA Choice Specs Magnezone Thunderbolt vs. 192 HP / 64 SpD Assault Vest Celesteela: 302-356 (78.8 - 92.9%) -- 37.5% chance to OHKO after Stealth Rock
Regarding the things that it can "eat hits" from, it'll only be safely switching into Life Orb Greninja's HP Fire once before it gets taken out. The same applies to Landorus a lot of the time. And it definitely won't be appreciating a Life Orb/Choice Specs Fire Blast from Hydreigon. I do understand that this was rushed, and I usually make this mistake too, but I do hope that your next nomination will go further in detail and perhaps bring up more relevant reasons as to why a specific Pokemon should rise a rank, especially for S ranks.