MasterOLightning
Optimator
If you can't see why it's not groundbreaking to combine elements from two fully realized genres, then I can't help you. Heavy metal bands had included prog elements in their music long before Opeth.had the exact same question ... before Opeth there was no other band like this, and there still isn't ...
no other band in metal combines these kind of influences to the same degree (although about the same time with Opeth some other bands were).
how much more does one have to do to be groundbreaking??? invent new instruments, invent new notes, invent whatever ...?
together with some other bands, Opeth have brought a new vibe to metal and have made a bridge between progressive rock and extreme metal, which is something no one could have imagined early nineties. look at websites and forums of progressive rock lovers, they embrace Opeth, whereas they used to despise death metal.
Burzum, Darkthrone, and Atheist are bands that were actually groundbreaking. Or maybe Carcass, who helped pioneer three subgenres.
By the way, Atheist, Death, Cynic, and other bands were plenty progressive well before Opeth's first album. They took core death metal ideas and made them progressive, while Opeth simply added prog rock ideas to death metal. I hope you can see the difference.
I think you'd have a really hard time finding any truly groundbreaking metal bands later than the mid-90s. That's not to say that there aren't creative, excellent metal bands making music currently.