How come every single thread about MM issues is locked?

Because it is 'redundant'.
I certainly do not agree with the locks. Players have a right to complain about issues in game. 

Every single one, eh?

I've often seen moderators consolidate threads, moving a new one under an existing thread on the same topic. I'm not sure if that's practical in this case, but it might be worth considering.

And when you get complaints that there are too many, that they are crowding out the useful threads, that the forum quality has declined as a result, and that they toxify the atmosphere, and some of your established regulars are indicating they'll participate less... what do you do then? 

Serious question, I'd be curious as to your input! 

Have the regulars contributed to the toxic atmosphere? As someone who was new and not a regular I have to say yes. They were extremely dismissive of the complaints early on, as well as had a tendency to condemn people, as clubbers therefore deserving of the pain, or arrogant 「multiple dookie delivery chute」, telling people they lacked real skills thus needed to learn to play. By doing so they only aggravated people with legitimate concerns. That's not helpful right now anyway, so actually if they're just lurking these days it's probably the best thing they can do, outside of still posting the occasional piece of advice. In fact, if they can just ignore the MM comments and keep posting helpful stuff, or friendly non-MM banter they can lead the newcomers by example.

Will they come back once things detoxify? Yes. I believe they will. They're regulars and I'd assume many are loyal to both the game and the forums they've been a part of for a while. After as much as they've already invested they be foolish to not have some patience for a bit. This will fade.

Is the forum quality a real concern? Prior to being linked in game I would have said the answer was subjective at best. However, now that said link is in place the answer is a firm yes. With that in mind, if I were vested in this forum, I'd be seeking options at every opportunity to restore what once was.

With that in mind how bad is the atmosphere really? Compared to before the MM change, it is worse, definitely, but when I consider the old clubber threads I don't think it's a total breakdown. Comparison helps too. How does it compare to the War Robots Facebook page? This forum is way, way better IMO. I've found the War Robots Facebook community less than functional. I think it's also important to remember that with the in game link there's probably some trickling over that are from the War Robots Facebook community, which I find far more toxic. At that point I tend to think a slight uptick in thread reminders to stay on topic, so people come to terms with your expectations might be helpful. I do understand that you don't want to be, and shouldn't have to be babysitters, so it's a tightrope you're walking.

What needs to happen to restore it's former self? First, War Robots needs to fix the league interface. Until then too many will focus solely on the MM, which people just cannot seem to let go of on both sides. We can only wait on that. With that in mind the relentless cheap shots, on both sides, that are prevalent even in threads not about the MM (These in particular are where I feel you can help the most. Kindly flagging people for instigating the unnecessary, unprovoked cheap shot at the opposition may help diminish those and then at least the fires will be contained in the proper threads.).

There needs to be acknowledgement on both sides that the other view point has validity even if someone disagrees with it. You probably haven't noticed it, but that's the direction I have been trying to move recently. In example, if my perspective is at least acknowledged as valid it's harder for me to keep my personal fire alive. I'm not sure what a moderator can do about that, but I feel it's important.

You're in a tough position. This forum has been made more visible at an inopportune time. It would have been far better to have been linked either before the new MM hit, or after the league interface was resolved.

I do want to remind you that there have been people that started out fairly toxic that are now posting thought out comments, even if others don't agree with their perspective. They're prime examples of people who vented and have started coming to terms with what cannot be changed. I believe they prove my opinion that most people will eventually run out of steam and normalize. The more I think about it the more I believe that containing the unnecessary cheap shots in unrelated threads is important. It's frustrating to read a thread about x, or y and find your MM perspective being crapped on yet again, unprovoked. Those comments keep the kettle simmering, so to speak.

I'm clueless about some of this social stuff, so please take my opinion with a grain of salt. The last thing I want is for anyone to think I feel like I know more than I do. I appreciate your listening though. Thanks..

Just for the pure irony, can someone lock this thread XD

Can ppl stop tagging me in things that don't involve me? Thank you

Because hearing people whine about something that will not change anytime soon gets tiresome.

Well said. But how do you know if someone has necroposted without opening a thread?

The mobile site is not very friendly.