I agree with some of what you're saying, but it's largely part of why I stopped actively seeking out new music and engaging with people strictly on the basis that we might share similar attitudes about bands or whatever. I got so tired of the response to metalcore and nu-metal over the years that it completely alienated me, and I don't even care about either. I don't know when it became cool to respond to people's ignorance of certain bands or styles with outright hostility, nor do I get the need defend more legitimate forms of metal with whatever might be mistaken for it in the mainsteam. It is so insufferable to see a Lamb of God thread and know that people with nothing more than an abundance of free time and a high opinion of themselves are going to shit on the band or fanbase for some completely arbitrary reasons. The attitude that 'real' metal needs to be defended in the face of 'false' styles is horrible, it really is. For that matter, it's horrible any time someone conflates their dislike of something with that thing's fanbase. It's not enough to hate Slipknot or Lamb of God, it's about the kind of people who like their music or what they supposedly stand for. My biggest take away from your post is there are certain attitudes and behaviours from younger fans of the genre, but that's there no pressing need to really be any different.

I've never seen the trivializing attitude towards the classics of metal from anyone other than total newcomers to the genre and from metalcore/deathcore/nu-metal

And that's the problem. That attitude is inherent everywhere, but the only difference between outright shitting on classic and formative metal and shitting on metalcore, no matter how much you don't want to admit and how much I'm sure our tastes align, is that you happen to think one is shit and the other isn't. That's it, that's all. People need to stop conflating all these issues and generalizations with what any individual might actually think. If metal means having a good idea of what kind of person they are for having arbitrary taste, then this is a fine lesson in why I keep to myself and just find new albums on my own.