Actually, Celtic Frost and Bathory were mainly protagonists of Black Metal, although I wouldn't doubt their influence on death metal
And technically, Possessed's "Seven Churches" which came out two years ago is the starting point of death metal, I think. I know that them and Death were two of the first DM banRAB.
Nevertheless, all hail Death! AWESOMENESS! rip Chucky.
And I think Opeth gets associated with Death Metal mainly because of Mikael's death growling (which is superb). Sure, they don't have complete nonsense fast drumming and sweep picking, but I wouldn't rule out death metal as an influence on their music.
Then outsource your listening to deathcore to little Indian children so oyu don't have to deal with it...... that's how Best Buy did it with their online shopping tech support.