Although you have a good point about how we shouldn't ban something because it doesn't bring much to the metagame, for unbanning it is a little more complicated. The reason why people say "what does XYZ bring to the tier" is because it needs to be compared to what it won't bring to the tier. What I mean by that is will the overall impact of a pokemon be a positive or negative influence on the metagame. Sure, a defensive pokemon brings a lot to the tier and checks a lot of threats, that's cool, but if it means that picking any other defensive pokemon is going to make your team much worse or the metagame has to use otherwise mediocre pokemon to beat it over better overall pokemon, that isn't good.Let me be real
"What does XYZ bring to the tier" is about the stupidest point one can make that isn't relevant to solely one specific mon. It implies that we keep things OU not because they aren't determined to be broken, but rather because of an overarching design in what we want the metagame to look like. As far as I'm aware, we don't tier based on vibes, but if that's changed since we banned Naganadel, do let me know.
Usage, the value itself not important whether it be 3.5% or 4.6% or something else entirely, determines if something rises or drops, and unless it's a fucking monster in one tier and utter shit in the tier above it, we don't change what tier it is unless its usage warrants us to. When it is that unique case, we vote on if it should be banned, and if it's voted out, we don't make it the tier above just because, but stick it on the banlist, like reasonable people, by considering whether or not it's real uncompetitive, be its ability to OHKO fucking Blissey with a special attack, or its ability to 6-0 a team weak to it, or the move in question making the opponent's attacks never hit, ever. Yeah I'm a little salty about Kyurem's ban.
At no point did we, or should we, ask what the thing in question brings to the tier. It's a question that implies that the thing isn't broken, but rather just undesirable to some vocal people, because its never asked as a first question. It's always a last resort, asked so that whatever the answer may be, the asker can say that, "the things it brings are covered by mons in the meta already," to paraphrase. Such an answer implies that not only can we have one single way to do those things, but also that the mon being asked about is assumed broken from the get go. This isn't to imply that we should think of literally everything as Not-Broken from the start, because believe me some things are just too much even for Ubers, much less here, but rather that we shouldn't jump the gun and keep things banned because they were banned once before.
Who gives a shit what Blaziken brought to Gen 8 OU, when it wasn't even good enough to end the generation in OU? It ended in UUBL I had to check.
Point is, actually argue on whether or not something is broken, rather than waffle on about how, "oh it doesnt matter what volcarona brings since its defensive utility is covered by other things, wheh."
P.S. how the fuck do i change my name dynamax isnt even a thing anymore
Edit: thanks kosecant
We can't know what pokemon will be broken, we can only infer. But until the metagame calms down after the initial wave of bans, very few pokemon should be unbanned to not have another crown tundra crisis. It may be the prime time to test these pokemon, but it doesn't mean that it will be good for the metagame. We have to think both short term and long term. Focus on one too much, and the metagame becomes horrible.