It seems to me that the main point of your case is giving a Pokémon a chance back in hopes of it changing anything, while ours is substantially improving the metagame by getting rid of extremely unhealthy interactions. Lets not forget the invitational is coming up, as well as SPL and I doubt we'd get Mega Gengar suspected in time for any of those tournaments at this point. It's about priorities, in my opinion. Also, keep in mind DD Zygarde is as common as ever, Substitute is literally everywhere and, as far as I remember, set up wasn't really that huge when Marshadow came around, the issue was how it limited that option to begin with. It's really not that linear and you'd just be including more unhealthy interactions to a metagame that already struggles with those.I haven't been very involved in DOU since this DPL so maybe I'll come off as uninformed, but one thing that's bugging me about all this support for a Gengar suspect is that none of these reasons actually convince me that a Gengar test should precede a Marshadow test that is seemingly in the cards. Don't get me wrong, I do think the opinions on Mega Gengar are pretty clear and people want a Gengar suspect for good reasons, but that seems exactly why a Marshadow test should come first. As far as I can tell this meta isn't a wildly heavy setup meta that would be flipped on its head with Marshadow's reintroduction, and it is a Pokemon that does have its strengths against Mega Gengar (and just as importantly, its weaknesses), so it does have potential benefits to bring to current meta in addition to being a somewhat controversial ban in the past. I don't currently play this meta but I'm no stranger to this meta, and I do think there's plenty good reason to retest Marshadow at some point.
Yes, there is a lot of support for a Gengar suspect right now from top players after several months of this current meta and I think that's an important point, because in the past this meta has suffered a lot from meta-relevant Pokemon being slowly introduced after previous Pokemon are banned. The fact that Gengar is still getting called for another test at this point convinces me that a Gengar retest should happen. However, I know a few people like GenOne have commented many times on how constant addition and removal of meta elements has resulted in us playing an incomplete meta and I think this is a point that gets overlooked too often, and actually supports a Marshadow test before a Gengar one. I'm not saying that the introduction of say Intimidate Incineroar is enough to make Marshadow "Not Broken" but I think it's a much more convincing point in favor of a retest than "this meta has had plenty of time to develop and Gengar is still the best Mega and I think it's broken." Oversimplifying yes but you get the idea.
Now I do think Marshadow gets unfairly demonized by this community and so many will disagree with my opinion on Marshadow, but the meta we have now is different than the meta we had with it previously, and if it's going to be tested I think it makes more sense while Gengar is still in the meta. There's a number of scenarios that arise from a Marshadow suspect and I think they're all beneficial in the long run.
- If Marshadow's presence is deemed acceptable and it has a positive effect on the meta that tones down Gengar, it's a very clear win scenario. To preempt the inevitable question, this is different than the "broken checks broken" fallacy (which sometimes isn't even a fallacy but that's a different discussion), because if we're letting Marshadow back in that assumes that enough of the community thinks Marshadow is not broken.
- If Gengar is deemed still problematic even with Marshadow around, I think that's even more reason to test Gengar shortly after because clearly having another strong Ghost-type isn't impacting the meta enough to hamper Gengar's presence. Another win scenario.
- If Marshadow remains banned then we can set the Marshadow question to rest permanently (win scenario). Most importantly, Marshadow remaining banned still allows for a Gengar test, ideally shortly afterwards (win scenario). We also avoid the very real issue of a Gengar retest feeling like a mulligan (a win for the council), which I don't think people are respecting enough. I genuinely think this community has an issue with "mulligan" scenarios that are unfairly blamed on "randoms that sway the result" more often than it actually happens. I'm not saying it hasn't happened before though.
- The remaining scenario is that Marshadow is unbanned and people think it's broken, but this is a faulty scenario that implies this community is really going to unban a Pokemon they think is broken.
I think any scenario here benefits the community in the long run so I feel like a bit more patience pays off the best. I'm not saying all this because I really want Marshadow back or don't want Gengar banned, in fact I would probably agree with banning Gengar. I've seen a lot of posts disagreeing with the council decision to not to want to test Gengar next though, and I respect that opinion, but I think there are more benefits to a Marshadow test regardless of the outcome.
Either way, putting my opinion aside, do you really want every round of the Invitational to be either a Mega Gengar domination or a mirror decided by a single speed tie? I know it's Halloween, but we don't have time to play around with ghosts.