I think Carracosta is fine in A-... It handles physical attackers really well and sets up Stealth Rock. But in practice that's usually where it stops. Scald is cool, Aqua Jet is cool but nothing amazing. In reality it's only going to pick off severely weakened Magmortar/Pyroar/Archeops. It's nice to have but once again its usefulness is just limited. Carracosta also faces a lot of competition as a SR user and is thus limited to teams that are particularly weak to the threats you mentioned. Otherwise you just use Uxie or Archeops or something because they're flatout better mons all around.
You're really downplaying the usefulness of Scald and Aqua Jet, especially when their qualities are unique to Costa for a bulky SR setter, which thrives on the utility they have; I don't see how spreading burns and packing priority to pick off weakened threats isn't appealing. I don't know how you've been using your Carracosta but it has consistently put in work against a large majority of physical attackers due to its bulkiness and various ways it has of retaliating, via Scald burns, poking with Aqua Jet, or simply just toss out a Rock attack (your strongest attack). Uxie and Archeops most certainly do not compete with Costa as an SR setter because they are just completely different in their roles (Uxie is more for pure support while Archeops takes the early-lead advantage), Costa's main competition would be Rhydon and Regirock. While Costa has less sheer offensive presence than Rhydon, Carracosta has enough perks, most notably Solid Rock + Leftovers, to put it on par with Rhydon, at least in this meta. Regirock has also improved in this meta but it being such massive Ground fodder (or greater fodder in general) is kind of off-putting.
252 Atk Life Orb Tauros Earthquake vs. 252 HP / 0 Def Eviolite Rhydon: 135-161 (32.6 - 38.8%) -- 99% chance to 3HKO
252 Atk Life Orb Tauros Earthquake vs. 248 HP / 192+ Def Solid Rock Carracosta: 109-131 (31 - 37.3%) -- 0% chance to 3HKO after Leftovers recovery
252 Atk Choice Band Sneasel Icicle Crash vs. 252 HP / 0 Def Eviolite Rhydon: 192-228 (46.3 - 55%) -- 62.5% chance to 2HKO
252 Atk Choice Band Sneasel Knock Off (97.5 BP) vs. 248 HP / 192+ Def Carracosta: 118-139 (33.6 - 39.6%)
252 Atk Archeops Earthquake vs. 252 HP / 0 Def Eviolite Rhydon: 132-156 (31.8 - 37.6%) -- 90.9% chance to 3HKO
252 Atk Archeops Earthquake vs. 248 HP / 192+ Def Solid Rock Carracosta: 106-126 (30.1 - 35.8%) -- guaranteed 4HKO after Leftovers recovery
252+ Atk Kangaskhan Earthquake vs. 252 HP / 0 Def Eviolite Rhydon: 112-132 (27 - 31.8%) -- guaranteed 4HKO
252+ Atk Kangaskhan Earthquake vs. 248 HP / 192+ Def Solid Rock Carracosta: 90-106 (25.6 - 30.1%) -- 0.9% chance to 4HKO after Leftovers recovery
252+ Atk Toxic Boost Zangoose Close Combat vs. 252 HP / 0 Def Eviolite Rhydon: 226-266 (54.5 - 64.2%) -- guaranteed 2HKO
252+ Atk Toxic Boost Zangoose Knock Off (97.5 BP) vs. 252 HP / 0 Def Eviolite Rhydon: 91-108 (21.9 - 26%)
252+ Atk Toxic Boost Zangoose Close Combat vs. 252 HP / 0 Def Rhydon: 338-398 (81.6 - 96.1%) -- [guaranteed 2HKO after Knock Off]
252+ Atk Toxic Boost Zangoose Close Combat vs. 248 HP / 192+ Def Solid Rock Carracosta: 181-214 (51.5 - 60.9%) -- 94.5% chance to 2HKO after Leftovers recovery [at least Costa doesn't get KOed after a Knock Off, and can still hit -1 Goose with Aqua Jet in a pinch]
As you can see, Carracosta handles the top physical threats in the tier
better than Rhydon would.
So you have those cool perks, but then you also have to deal with plenty of flaws. Carracosta is weak to Electric, Ground, Fighting and Grass. His weaknesses are so common that Weakness Policy became a standard set, and that says something. Seismitoad was so ubiquitous because you only had to cover for one weakness. Carracosta users have a lot more weaknesses to cover for which makes it much more difficult to fit on a team. Yeah Toad is gone now but that doesn't mean Carracosta is now the next best thing for sure considering these flaws.
First off Weakness Policy is not very standard, as Life Orb or White Herb are preferred. Secondly, having several weaknesses won't be that much of a bother if they are easy to cover (a Grass-type alone handles like three of those weaknesses), and that didn't stop Rhydon, which has even more weaknesses, from getting A Rank. In fact, Costa is blessed in that it rarely risks exposing itself to its weaknesses (unSTAB Earthquakes are easily soaked up for the most part), making it harder to lure, while Rhydon's Water weakness is enough for people to take note of and prepare such lures as Aqua Tail Archeops and Kangaskhan, which can greatly hamper Rhydon's ability to check them, much less anything else afterward; Costa does not face nearly as much risk since Grass coverage is not easy for physical attackers to come by.
Then there's its awful special defense. It's just bad, and special attackers are by no means uncommon. People use Rhydon because it can tank hits from both sides of the spectrum and isn't weak to Electric. So there you have why Carracosta is below Rhydon.
People mainly use Rhydon because it packs the most offensive presence out of all the bulky SR setters. Also Rhydon's enhanced special bulk from Eviolite is mainly for Fire-types; if you glance at the mons in the higher rankings, there really aren't much of any special attackers that beat Costa but lose to Rhydon, because Rhydon stands a good chance to lose to them anyway. So you can't really say that Costa deserves a sub-rank below just because its Special Defense is lower, because for the most part Rhydon's Special Defense doesn't help it that much more against most special attackers.
Also:
252 SpA Life Orb Pyroar Hidden Power Grass vs. 248 HP / 68 SpD Solid Rock Carracosta: 292-347 (83.1 - 98.8%) -- guaranteed 2HKO after Leftovers recovery
And yeah I know Carracosta can use Shell Smash but let's be honest here; Barbaracle, Huntail and even Gorebyss do it better. I've used it plenty, I even used special SS Costa back when Tangela was on every other NU team but no matter how you turn it you're just using an outclassed sweeper. Which is a bad idea even when you're trying to lure certain mons or going for the surprise factor.
Carracosta imo is probably
the most consistently useful Shell Smash sweeper in the tier, and that is mainly because it is capable of contributing early-game or even contribute at all if it doesn't manage to Smash, unlike the other Smashers, due to its high physical tankiness and access to priority which lets it go toe-to-toe against threats even without its boost, instead of sitting its ass back until mid/late game before finally showing its face if it doesn't want to just flop over and die (looking at you, Barbara >.>). Not only that, but Aqua Jet also helps Carracosta greatly in the midst of its Shell Smash sweep, and is the biggest reason it can afford Adamant to further fuel its power, while letting it hit Scarfers and faster mons (most of which are too frail to eat a +2 LO Aqua Jet) and evade priority moves, most notably Sucker Punch. I didn't (and won't) touch upon the specially mixed Smash set because I don't have experience with it and it seems like a cute lure at best (if you're going to settle with that much inaccurate moves on Costa
and give up Aqua Jet on it, you might just be better off with a Barbaracle). I really don't see how you can call Shell Smash Costa outclassed.