TL;DR: It's less of "no one complained about toxicity" and more of "there was no one to complain about toxicity due to event being fringe, and complaints poured in once it stopped being fringe".
There simply weren't complaints until people began to ask for "more inclusivity". Which I'll cover in your next point. But even in the two years before the rules came in and some speedrunners got hit by that, there was a lot of people coming and watching, and there was a lot of companies gifting money to the event. Which is why the comments about "toxicity" (which I still need to find proof for) surprised about everyone.
Speedrunning community was (and still kinda is) notorious for its degree of discrimination towards female runners, so concerns were pretty legit, even if approached not in the most laconic way. For "what matters was speedrunning and not who speedruns", you'll barely find any female runners (let alone any trans folks) on early GDQs, and not for the lack of willing participants.
There was no proof of such discrimination outside of tweets and discussions that have been confirmed as fabricated. The answer from the fabricators was to shut down their Twitter accounts because they were "menaced", which has no proofs and thus held no grounds. Feel free to post any of them if you think I'm wrong.
In addition, there is no care about the gender of runners. The speedrunning is the art of playing a game faster than anyone, and if you can run a game faster than others, it doesn't matter if you're a woman, a man, a transgender or anything else or in-between. Which is why the claims against gender equality also had no value. You don't need to have an equal number of sexes or genders to prove X can run the game faster than Y. And you don't need to care about being discriminated if you can run faster than both. Just post the proof and let the rest happen by itself.
For the most part, average audience doesn't want this kind of stuff, and as I said, since average audience already surpassed a rather small subset of "hardcore" speedrunning fans, GDQ makes a reasonable decision of hammering down on slur-throwing bickering in the chat and swear words on the air. Considering the $3M record, the decision paid off well. Runners are warned in advance, so it's mostly on them .
For the most part, average audience doesn't want this kind of stuff, but as I said, average audience doesn't care about gender inclusivity, gender equality or toxicity. They just want to have fun watching players speedrunning while having fun, which was part of the thing attracting so many people, and the fact that numbers are dwindling both in the speedrunning community and the people watching the livestreams is a glaring factor of it.
As for the $3M record, it is a logical consequence of what happened before - The more mainstream the event is, the more money has to be injected in it, for showing how generous you are. It's a shame, but it is a known fact, and the fact many charities thrive on support because they aren't mainstream enough reinforces that.
GBATemp threads aren't making for the greatest pool of people to confirm common opinions. Besides, GDQ still attracts a lot of big names and breaks revenue records, so it's hardly in decline either.
Revenue records are growing because of the mainstream effect, which I explained on the previous part, and the lot being attracted is complaining more and more about how the show lost its quality and all the fun they enjoyed having. So while it's hardly in decline, I'd answer it's hardly in decline yet. The moment a decent competitor will rise up and be as mainstream as GDQ, their inclusivity won't make them the most popular option.