This was rather inevitable. Back when moody was banned from Ubers, I made the argument that Ubers was a banlist, not a tier, and thus an ability shouldn't be banned from there. Once it was acknowledged that things can be banned from from Ubers, the logical conclusion was the Ubers would be made a tier. That also made sense as people actually played Ubers seriously, and there was no reason to not have people actually try to keep it balanced.
Now, back when generation 5 was released, Smogon was arguing about how to start tiering. I remember several proposals, and they ended up doing what we know now, which is banning a certain group of pokemon and then testing from there. At that time, oh so many years ago, I made the argument that a metagame with all of the pokemon that were quickbanned could be balanced, although one or two of those pokemon might end up being banned. It would, however, look significantly different from what OU traditionally looks like. It would basically be what we now know as the Uber tier, except with a banlist above it and usage based drops. Now, the decision made at the time was to stick with the status quo, which wasn't the wrong one, but brought up some interesting questions regarding what a tier actually was meant to be. OU wasn't supposed to be "all of the pokemon minus the one's which cause severe imbalance", it was supposed to be that generation's traditional OU, which would look like the OU of every generation past.
Now we reached another interesting point, where we've acknowledged that it is possible to try to make Ubers balanced. At this point, the next logical step would be to make OU based on usage in Ubers, but Smogon has said this isn't happening at the time. This makes sense - we wouldn't want to have to re-figure out all the tiers below OU all over again. However, come gen7, this could be a legitimate proposal.
The other thing we've acknowledged is that that the usage based tier system isn't sacred, as Ubers with low usage don't drop. Now this begs the question, why isn't this true for all tiers? If we are going to start by arbitrerily deciding pokemon for OU and Ubers, and working from there, why can't we take UU and try to balance it. As anyone who has played a lower tier knows, the metagame goes with the whims of the higher tier. If a pokemon stops being used, it could drop, and then you've got a whole new force which may be an imbalance. But if we aren't dropping Ubers into OU, why must we drop OU pokemon into UU. If UU decides that its balanced, why not leave that pokemon in OU until they feel like they need a change. The only was Ubers drop is by suspect test, but why not implement that for all tiers? Also, why not drop a pokemon which is OU but would also be balanced in UU (niche pokemon, for example)?
Part of the answer to this is what was known as the slippery slope, and was used as an argument back when Aldaron first proposed the Drizzle+SS ban. The argument was that, if we can ban something like that, why not ban Kyogre+water spout, or whatever, and try to make EVERYTHING OU - with certain limitations. However, it is understoof that at some point complexity isn't helpful. Having a high usage pokemon in OU be UU is confusing, and how this would be determined is even more of a headache - especially for something which would only be used once or twice per tier.
Now, I'm not trying to lobby for any change in particular. I rarely play Smogon tiers any more - I now stick to VGC. However, I just feel like these are interesting questions to ask, especially given the fact that we can change how the tier system works, and Usage based tiers are no longer sacred.
