I can’t really say that the slippery quality of discussion has gone down because discussion would be an exaggeration of things before more people jumped over. There just weren’t enough interactions between people to use as a reliable indication of quality.
Now, any forum I’ve ever been on, even back to ICQ days, and old school bbs stuff, there’s a certain degree of “poop in the peanut butter”. You eat enough peanut butter, and your chances of also eating bug poop go up, with it becoming a certainty eventually. You get enough human beings in the same place, you get assholes. The more people, the more assholes.
You can’t really escape it entirely. Even the best possible moderation has delays, and judgement calls. All you can do is drop the numbers until the amount of poop in the peanut butter is low enough to pretend it isn’t there. The only way to entirely get rid of the bad elements is to host your own forum, and make it invite only, while also not letting it get big enough you can’t moderate it.
But, my impression is that the good discussions are still way higher on lemmy as a whole than anywhere else currently. Mbin counts as part of that, though it isn’t the same thing.
Blocking instances helps. You’re right that some instances have a certain proclivity towards personality types and/or beliefs that go hand-in-hand with bad behavior. And the bigger ones (the one I’m on included) have big enough numbers that even without those proclivities, you end up with good bit of poop with your pb&j. Sadly, even blocking an instance isn’t going to fully prevent running across users in comments here and there.
But overall? Way better than reddit was in the five years or so before spez spazzed. Better than 9gag ever was. At least as convivial as one of the forums I moderated back in the late 90s, where the user base was pretty decent overall by virtue of it being homogenous in several ways.
As pissed as I was that reddit went all the way into stupidity, I’m happier on lemmy now than I was on reddit a decade ago. And I’m happier with the average interactions I see on lemmy than I was with lemmy before last year. And that’s including the truly abysmal run ins I’ve had over the last year here.