I'm not entirely convinced that Alomomola needs to take the roll of blanket hard walling the majority of physical threats in the tier to be justified of the S Rank status. Alomomola's bulk is by no means impenetrable, as evidenced by the fact that it does indeed have a relatively poor match up with a number of the fighting type Pokemon in the tier, but it's also significant enough that it can withstand a hit if need be an recover by sheer merit of switching or wishing. But that to me isn't what could propel Alomomola into rising into S Rank - rather, its the fact that it's an exceedingly low risk Pokemon in the following ways:
1. It does wall significantly dangerous threats in such a way that it can provide repeated support against what it does cover and end up relatively unscathed by merit of Regenerator. This makes it incredibly low risk to use compared to other walls in the tier who, even if they have reliable recovery, must use up their turn to heal and in turn provide a greater opportunity for the opponent to either a. switch in a direct threat that can harm the other team or b. forgo healing to directly address that threat but then provide greater opportunity for the walled Pokemon to break through. Alomomola has the option to opt to heal but also does not have to due to the leeway provided by Regenerator. And, as Spirit said, when it opts not to heal it still provides useful functions in Knock Off, Scald for the burn and Toxic, which whilst it can't run all of them simultaneously they do serve distinct useful functions against many of the threats. I personally see Knocking Off a Life Orb, Leftovers or an item other than say a Focus Sash where it's literately useless to be an incredibly useful function for a balance or stall team, as even if say a Sigilyph switches in and remains a presence the Pokemon that handles Sigilyph has a sustained benefit for the rest of the match. Status is very obviously useful, nobody argues with that, it in some way inhibits a function of the opposing Pokemon. It's worth noting Alomomola is primarily useful on teams of which rely on being able to handle these threats multiple times over the course of the game - anything that increases the number of times a threat either needs to be handled or can be handled directly serves the purpose of the playstyle.
2. Alomomola has access to Wish, which means that any time there is a weakened Pokemon on it's team it limits the opponent's options such that they must prioritize preventing the Wish from going through. Wish is very clearly useful in healing Pokemon that lack their own recovery, but it also serves the function of pressuring the opposing team that relies on the evisceration of these threats into adhering to strict patterns (ie, forcing in a Virizion every time as to pressure both Alomomola and the weakening Mega-Steelix (presumably within Virizion's Close Combat range)). Any time you generate a pattern in the opponent, you gain a subtle advantage because they must adhere to your pattern or risk a lot whereas you don't need to adhere to the pattern (Alomomola doesn't need to wish, it could toxic the incoming Virizion). This is not always relevant, but it's an extra factor aside from just 'Wish heals things that can't heal themselves' to consider. I figured it was worth mentioning as a potential strength of Alomomola, as it's particularly good at forcing these patterns due to it's general bulk yielding a need to break through it and the receiver quite effectively. Also, let's not forget Alomomola is a pretty decent Pokemon against opposing balance/stall due to the fact that they will have less capable of breaking through bulky Pokemon and thus preventing these wishes than offense will simply due to the goals of the playstyle (more of an emphasis on winning slowly by finding the weak point and wearing it down over time than aiming for direct pressure via a threat). Also, even if you switch in a dangerous threat, let's not forget that a passed wish can still make the incoming check/counter better at dealing with the threat.
3. Alomomola by repeatedly switching in to a threat with less risk of taking residual damage over time due to Regenerator slows down the pace of the game and can help readjust that pace in the favor of the playstyle. To explain this, let's set up a hypothetical situation. You have out Hoopa, You know I'd like to pursuit your Hoopa, so I'd send in my full health Specially Defensive Drapion. You could Focus Blast, but to be safer you predict this and send in a Stealth Rock setter Rhyperior. Hypothetically let's say you have rocks up, so you use earthquake, stone edge/rock blast, run the random ice punch to predict a Flygon (not recommended for serious games), perhaps Toxic, etc. Regardless of your options, I can send in my Physically Defensive Alomomola and render all options aside from Toxic of minimal effect, and whilst Toxic is certainly helpful it only weakens Alomomola once, so if this were to happen again you'd very literally not have any options. Whilst the Toxic is nice, the fact that Alomomola can effectively scrub clean mistakes in the manner that is unique to Regenerator Pokemon is pretty fantastic, and it's why Regenerator is such a good ability in RU.
4. Alomomola provides massive support in the team builder in that it can compress all of these useful roles into on Pokemon as well as provide the aforementioned general bulk and the critical water typing. It really is one of the best role compression Pokemon in RU right now, it can fill all of these slots at least moderately well (and some excellently). Whilst it may not directly counter everything and has distinctly important weaknesses to balance breakers like Sigilyph, the sheer density of rolls it does fill could concievably be argued to outwiegh the cons from the perspective of what it does provide for the overall health of the team.
I definitely think the weaknesses to Pokemon like Sigilyph, Virizion, Meloetta, Venasaur, M-Abomasnow etc. that do threaten balance is a significant factor. I also think that the choice between physically defensive and specially defensive yields a situation in which Alomomola often seems better on paper than in practice due to the fact that it must specialize. Not to mention it has 4MSS. I'd probably keep it A+ were it up to me due to it's rather significant exploitable weaknesses in the metagame. But I don't think the argument for S rank is centered around it's ability to simply 'wall dangerous attackers', nor is it to handle the threats of offense (certainly some, but not all). And I don't think it's entirely passive, because a momentum shift is a pretty big boon and whilst some Pokemon are exceptions between Knock Off-ing, Wishing or statusing Alomomola can still do something meaningful against many of these Pokemon. I think the fact that it gives a lot of legroom for balance to work with and opens up teambuilding slots compared to the alternatives in conjunction with just 'pretty good defense' is why it's up for S Rank, and I see a lot of people arguing against the 'bulk' and that it's 'strictly passive' when more is under the hood that is relevant than stopping X number of relevant attackers or 'only' removing a item/statusing. I think the fact that regardless of it's move can still provide opportunities for dangerous balance breakers is the most critical argument against S rank even if the turn isn't 'free' in the strictest sense.
