Yes - you have to remember the part that technology makes in creating new music.
The early bands (well - mostly 2nd wave - not typically the real pioneers) who used electric guitars, synthesizers and samplers, for example were the ones who tended to genuinely create new "genres" of music.
However, once we got to the position of high-quality sampling available to everyone down to the home studio level in the 1990s, it was suddenly possible for anyone to create literally any sound they wanted to. I genuinely agree with you that this means there simply won't be any truly new music genres.
The only technologies which could maybe be considered new would be things like using multichannel, directed sounds (eg. making use of a full surround sound image), etc. But I can only see technologies like that slightly changing the way music is delivered, not changing the music itself (I have a copy of The Flaming Lips, "Zaireeka", which is quite an interesting layered music experiment).
The only other thing I could think of doing would be breaking away from traditional western music into something microtonal, but plenty of people have tried that in the past with literally zero mainstream acknowledgement.
As such I agree that it's going to be nearly impossible for anyone new to truly create an image as big as those that have gone before. And I don't think it's limited to "metal". You can say the same about nearly any other genre - electronica, pop music, rap music, etc. There are big names in all these areas who are unlikely to be surpassed any time soon.
That said - it does come in waves - there's always space to revive out-of-fashion styles. Oasis (I liked their first album!) did a pretty job of making themselves known without actually bringing anything new to the music scene, for example.