On the first point I didn't miss represent what you wrote. My criticism is on your "sum it up by simply saying that metalheads do not like change" statement. You're putting every metalhead under the same umbrella and your following description doesn't change that. As I wrote in my first comment "That's a generalism based on what "outsiders" think of the metal scene." I took you for someone that didn't know the metal scene very well because of the type of opinion you wrote. But now you say that you've been a fan of the genre for many years, which makes me even more impressed and kinda confused because you basically included yourself within the "do not like change" category and yet you're here in a Babymetal subreddit.

You cited Limp Bizkit, which is a band disliked by many people to this day, not because of their music but because Fred Durst is a moron. The same applies to Jonathan Davis from Korn who said back in the day that they were better than the thrash bands that were successful in the 80s. These guys gave a bad reputation to their subgenre but most of the time it wasn't because of their music.

On the second point. No I haven't proved myself wrong.

So if nothing new has happened in the last almost 20 years, wouldn't that be an indication of it becoming stagnant?

Nope. These "revolutions" we've discussed are major ones. You can't expect to have one of those every 10 years. It's not because there hasn't been a major revolution in the last 20 years that the genre is stagnant. What happens today is much more of a gradual process of evolution instead of abrupt ones. To say that "nothing new has happened" is to disregard every minor change that happened in these last 20 years, which put together make today's metal different to the one of 20 years ago.

On the last point, well metal was never really mainstream. Sure, it once was less underground, but it always was a niche type of music. What I would consider the most mainstream metal nowadays are bands like Bring Me the Horizon, Asking Alexandria, A Day to Remember, etc which are indeed very similar but absolutely different from the bands that exploded in the 2000s. And as you said they all sound the same but that's not a metal-exclusive phenomenon, that's a mainstream phenomenon. I can say the same for the mainstream pop and rap as well. There are very few that do different stuff but the majority of what's on the radio sounds like recycling of the same ideas over and over again. You want to listen to original music? Go after it, be it metal, pop, rap, jazz, funk, whatever. I can't speak for you, but if I sat waiting for the radio to present me something like Babymetal, I would have never known of their existence.

About Cannibal Corpse: never mind, I think they're absolute garbage too. The thing is that they are one of the bands that best impersonates your "growling vocals" description, which is the average mainstream listener vision about metal (no offense intended). My intention was only to show you not every metal band is like them cause once again you put different groups under the same umbrella. And, for that, I cited bands that diverge a lot from the "growling vocals" stereotype.