The only problem with that logic is how rock and metal are two different things. Rock has an up beat and tense instrumental. While metal has a strong beat and intense instrumental. Metallica for example is Rock because it has an up beat which means it’s a little more of a beat than different music while also not being too strong like Metal.
ok, again.. metal has a VERY broad definition. norwegian black metal sounds extremely distinct from speed metal which sounds quite different from doom metal. compare Type O Negative to early Darkthrone. almost no similarity, but they're both metal.
Look up the documentary, "Metal: A Headbanger's Journey (2005) directed by Sam Dunn. He goes through the history of metal and all of its varied sub-genres. The bands you mention are there but they are only a small part of the big picture.
Not sure how long you've been a metal fan but I've been one since the early 80s and the bands that you call metal weren't around then. What I'm saying is that if you've seen that documentary, you would know not to limit what is and what isn't metal because the world of metal is wide and varied. Now if he had mentioned bands like Led Zeppelin or Van Halen, I would agree that they are just rock bands but Iron Maiden, Metallica and Megadeth are metal to the core and paved the way for those bands.
I'm an old school 80s metal fan myself and I think the main reason people don't remember the good metal bands of that era is because of the crappy glam metal bands that came out after the British invasion. People began to believe that glam was representative of the metal genre and the whole scene suffered from it.
some of those are still ongoing, but age does have a bit of a factor, can't make music if you're really old/dead. there are complications to that. but music has been failing for a good long time now, history listening wise, took place in the late 90s, been getting worse every since.