Saying Dark would be fine is one thing. Dream Eater Gengar mains Dark, and from what I saw with him, that is not the case. Not only would Fighting be significantly harder, but Dark would also lose another Fairy check, Water would now have an advantage against it (think about it, most of water is Special-Oriented, so Mandibuzz is not a challenge to take out, TTar is weak to water, Sableye is just about the same as Mandibuzz, Darks 3 best Megas can't do a lot against it, and Water is notirious especially for carrying a lot of ice coverage, so Hydreigon can also be checked), same for Electric (once TTar is out, you have the advantage essentially). So, in conclusion, yea, I think Dark would be affected, and its viability would drop a considerable amount (refer to stats after Greninja was banned, and before Hoopa was released).I was going to bring up how Hoopa-U would affect its own types if it left, but I didn't have time and thought that there was no point (in the end the ban will come to the Hoopa itself, not how Psychic and Dark will be affected), but Dece1t pointed out (see quote) what I was going to say about Psychic: It will lose nothing. I've been laddering
badlywith a Psychic team (one less Dark type to deal with), and I've realized Psychic has so many viable offensive options to replace Hoopa-U (I've heard Meloetta is doing quite well at replacing it, although I prefer Choice Scarf Gardevoir) that it won't lose anything from the ban, should it happen.
Dark however, will be hit harder. Dark already lost a big defensive threat in Mega Sableye, and should Hoopa-U be banned, it will lose a big offensive threat. However, I feel that Dark will recover. Dark, like Psychic, has a lot of good offensive threats, and while they may not do their job as well as Hoopa-U does and it may lose a bit of viability as a type, it will not be hit incredibly hard (like Ghost was).
As I said at the start though, the ban will not come down to these things, but it is good to bring them up, and although I feel both Psychic will not lose too much and Dark won't be completely unviable, I still stand by my opinion that Hoopa-Unbound shouldn't be banned, and at this point it will take a lot to change my opinion.
EDIT: While laddering with the Hoopa-free Psychic team, I couldn't help but notice something about Dark teams. Out of the 45 or so battles I did, I fought two Dark teams, which adds up to about 4% (slightly under last month's usage of 4.9%), which shows that Dark teams don't seem to be hit too badly.
Another thing I noticed is that I won against both these Dark teams with my Psychic team. According to the matchup table (which I inserted in the post; I'm not very good at just sticking images in) from March 2016, Psychic has a 39% chance to win. It is important to keep in mind that both of these turned up around 1300, and that this was before the Sablenite suspect. It wasn't too easy to win but I won against both of these which either shows that either I'm really lucky or that without Hoopa-Unbound, Psychic has a better matchup against Dark. If that's the case, it's a bit worrying. Psychic is getting stronger and even Dark, the type that's meant to take no damage from their attacks, can't even stop it.
Gonna say what I said before, the ban will not come down to type effectiveness, but just pointing this stuff out.
See even if considering that, I don't think Hoopa falls under that margin. On paper there are Pokemon that are also widespread that could very well be considered more threatening than Hoopa depending on the curcumstances (I was actually talking about that very topic with a friend earlier, we were talking about how excadrill + its team support can actually seemingly be more threatening than Hoopa-U + what team support it has). Even with that, despite Specs Hoopa-U has in theory no switchins, it is a lot easier to check than other wallbreakers. You do come up with a good point, which in a sense I do agree with you, but I personally don't see the huge hubub other than that 1 fact (that fact that it can break anything in theory). I wouldn't agree with that being the reason why it should be banned, because again, there are pokemon that, with their team support, can actually be seen as more threatening.Hoopa-U's ban would slightly nerf Psychic, but I agree that it would likely remain a higher tier type. This suspect is not about nerfing Psychic nor promoting diversity though, and it never was. It's about whether or not [primarily Choice Specs] Hoopa-U is broken. It sucks, but even if Hoopa-U is important to Dark, if it is determined to be "broken" on Psychic, that reason trumps keeping it on Dark, imo. With Shaymin-S and Kyu-W, we decided we wouldn't keep powerful Pokemon just to help out lower used types, I don't see why that wouldn't apply here.
IDK, it just seems like this suspect is just turning into a big question of "Does Hoopa-U bring an unhealthy element to the meta, and/or is it broken? If so, should it be banned, or should we keep it in order to salvage Dark's viability?" ofc I might be wrong, but the second part of that question is overshadowing what voters REALLY need to be asking themselves. "Does Hoopa-U bring an unhealthy element to the meta, and/or is it broken?" If the answer to that question is a yes, it should be banned.