Look, I'm not denying that many bands released good material during the 90's, and I also never stated that all of them surfaced during the 90's either--a handful of them would not have survived though if not for already being established during the 80's.
The fact remains that your list is actually pretty damn small and pretty much inconsequential in the grand scheme of things as far as metal is concerned and influential bands and albums go. I find it odd that you're in your 30's and don't already know this. Did you not start listening to music until you were in your early 20's?
I've been listening to metal/hard rock since I was 5 years old. The problem with your argument is that you only see bands who influenced the very small music spectrum you listen to, and consequently, most of those bands are from the 70's and 80's. You don't like death metal, you don't like black metal, you don't like doom metal, you don't like most forms of extreme metal, so what does that leave us with? Classic, prog, thrash, and power metal, all of which were influenced by 70's and 80's prog or NWOBHM or thrash metal bands. It's easy to say 70's and 80's rock and metal were the most influential when the only music you like is a derivative of said genre.
I'm looking at it from a modern perspective. I grew up on the Big Four, Queensryche, Maiden, Zep, Pink Floyd, AC/DC (who I can't stand anymore), Sabbath, NWOBHM, and lots of radio-friendly bands from the 70's/80's like Boston, Journey, Foreigner, etc. However, out of those bands who do you think influenced more metal bands today? That's right, the Big Four, and of those four, maybe only 2 or 3 influenced a large amount of bands because Anthrax was like a niche within a niche.
Honestly, if you're under the age of 35 and are listening to more bands and different music than you were 25 years ago, chances are you were more influenced by albums put out in the 90's. That's all I'm saying.
In the 90's, there was anything you wanted, from grunge to thrash metal to alternative rock to death metal to black metal to power metal to stoner metal and so on and so forth.
No one in black metal has written an album like Anthems to the Welkin at Dusk since.
No one in thrash metal has written an album like Rust in Peace or Far Beyond Driven since.
No one in death metal has written an album like Symbolic or Domination or Tomb of the Mutilated or Legion or Heartwork since.
No one in Power/Prog/Melo-thrash has written an album like Dreaming Neon Black since.
No one in doom metal has written an album like The Silent Enigma, Turn Loose the Swans, The Carnival Bizarre, or Alternative 4 since.
Also, no one has written an album like any of the aforementioned albums before the 90's either.
More bands have done it faster and more extreme but not necessarily with the same impact these bands had.
We can go back and have a pissing contest all day about who influenced the most people, but then you'd end up with various blues artists who were ripped off by Led Zeppelin. That's not what we're talking about.
