A few things first of all. I'm making a lot of arguments in this post assuming Quaquaval can find a good setup opportunity and handling it might require a free switch-in. This isn't always true, so don't assume I'm saying saccing mons to it is always the best option. Second, I think it's often worth asking "Would another Win Condition be dangerous as well in this situation?" Consider pokemon like Hawlucha, Revavroom, Slither Wing, Salamence, Trailblaze Zarude, Trailblaze Ogerpon-Cornerstone. Of course, Quaquaval and these pokemon have their differences, especially when it comes to their typing. However, in a Tera metagame especially, the typing will not be the main factor in matchups, but rather the mon's role. As such, their ability to win or lose a certain matchup ends up being somewhat similar in certain cases is what I've found.
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About "Checks"
Trying to take on Quaquaval with mons like Slowking, Rotom-Wash or Hydrapple is just the wrong way to go about it in my opinion. All three of these are NOT walls, they are pivots (and also a wallbreaker in Hydrapple's case). They're meant for positioning, taking somewhat weak hits and then switching out. When you switch your Slowking on a Latios or a Keldeo, it's not expected for either mon to die, simply to switch out. As such, Slowking only uses tools that make staying in awkward such as Scald or Thunder Wave, not tools that allow it to perform well in extended stay-in situations. However, for most win conditions, including Quaquaval, it is expected for one of the two mons to die and likely for the other to take significant damage as well. Win conditions are much, much stronger at staying in than pivots are and they will lean into that aspect further with options like Substitute, Taunt or Lum Berry in order to get a lot of advantage against them. The fact that Slowking, Rotom-Wash and Hydrapple have a type advantage against Quaquaval matters little as they're not meant to take on boosting threats. It's not just against Quaquaval that these mons struggle, they tend to lose to most mons with setup moves. While they may be a bit more specific than Quaquaval is, pokemon like Hawlucha, Revavroom and Slither Wing can be similarly devastating if not more against teams that mainly use defensive pivots rather than walls. I'm not saying that defensive pivots can't be useful against it, but I think it's completely natural for them to be somewhat poor options into a win condition.
About Bulky Playstyles
I've also seen talks of bulky playstyles not being able to fit walls to Quaquaval. Except... that's not true. It's just that these walls are niche mons that would perform poorly outside of bulky styles. However, when we're talking about actual stall, this is the kind of playstyle where we can expect to see mons like Avalugg and Slowbro, mons who actually have specced their bulk and moveset to take on high damage threats. Certain HOs can find themselves with three or four mons outright walled by a Tera Avalugg or Slowbro thanks to options like Iron Defense. This is the kind of move walls are capable of using while pivots can't. They're less adept at switching in and out, instead focusing on being as capable of surviving as possible. This is why they're rarely seen as pivots are much better in a metagame dominated by Tornadus-Therian, but if you want to take on win conditions with a defensive pokemon, walls tend to be much, much better at it.
About Balance
As for more Balanced teams, not running proper anti-offensive tools is just asking to get ran over by not only Quaquaval, but most Speed-boosting threats. Regardless, it's not like slotting these mons is hurting your team too much. Lokix and Deoxys-Speed particularly are not only capable of revenge killing Quaquaval, but also many other mons and they also happen to be some of the best progress-making options in the tier for BO and Balance. Skeledirge also shuts down a lot of offensive threats while being a great win condition. Sure, a well-played Quaquaval may still do some damage, but you should have the tools available to make playing around it very realistic and this is just naming the very best options (that can probably fit on 90% of balances and BOs anyway). Ogerpon-Teal and Scarf users like Latios and Zapdos-Galar can be your main backup against offensive threats as well and, even if they are not all that good at switching into it, common options like Rotom-Wash and Tornadus-Therian make it much harder for Quaquaval to find a good setup opportunity.
About HO
I'd also like to say banning Quaquaval would probably not hinder HO much. The style always shifts and adapts, changing its structure slightly to include a new mon that fits a similar role. We can already see plenty of offensive teams that use other speed-boosting sweepers over Quaquaval, other Fighting-types over Quaquaval, other Water-types over Quaquaval and they're not made any less dangerous by it. Mons that underperform against HO like the aforementioned defensive pivot continue to struggle. Mons that can shut down offensive options such as Deoxys-Speed and Lokix continue to be at the core of the game. As a result, teams that struggle with Quaquaval HO will also likely struggle with post-Quaquaval HO. Quaquaval itself is also good at applying pressure to offense and can fit on BO, so we'd lose out on an option that's generally better into offense which I feel would make the overall result even.
About In-Game
Now for the dreaded in-game argument. Quaquaval is not a mon you can simply throw your counter at and beat. It's a mon that you fight even when it's off the field. This applies to every mon in the tier, but for win conditions especially. Making sure that, if it gets in, it has as few options as possible to force progress and especially to snowball is key. So like if there's a Quaquaval on the opposite team, you gotta be smart about how you use your Tyranitar, especially if you lack proper anti-snowball options. Next, there's also a lot of decision making when it comes to what you do when it actually hits the field. What do you switch-into first? Do you scout for coverage? Do you use your Tera? Do you try to make your opponent use Tera? Can you do something to prevent Aqua Step or Swords Dance? In most cases, none of these decisions comes down to whether you can read your opponent's set right or to 50/50s. This kind of positioning, especially against HO, requires the ability to look ahead and see what you'll need for the remainder of the game. In a fair amount of cases, you have enough of a lead to just straight up sac a Clodsire against Quaquaval to prevent a speed boost. In others, you may have to play it differently.
Either way, the result is that Quaquaval leads to complex decisions. Not only for the one playing against it, but also for the one using it as positioning it and deciding how you wanna use it requires similar skills. This is so much more interesting than a metagame where you can just go into a counter, switch out and repeat. Obviously that aspect isn't completely gone without Quaquaval, but I don't think it having any hard counter is an issue when it's still very manageable with decent playing.
In short
I believe that beating Quaquaval, even without any hard-counters, is very realistic. It's just about knowing what your mons can and can't do, about having insurance in case it gets a good opportunity and about limiting its opportunities in the first place. Your options to keep it in check are more than good enough, provided you're fine with not having a hard-counter to it. I believe it just acts as another win condition that's available to both HO and BO and is not a step above the other options available. I don't think banning it would ruin the tier or anything, but if we want to ban something, I believe it should be because it is actually a problem for the overall health of the tier.
