It bugs me when they lump groups together because two dissimilar bands just happen to share members, or when they're considered similar because of historical association or genre fame. I mean, why should Black Sabbath only have five doom metal bands listed on the first four pages of artists they're similar to, while Dio, Pantera, Motorhead, and Slayer are all on the first page? Fail.

Depends on what you would call drone. John Cage's String Quartet in Four Parts is some very droney modern classical and it was composed in 1950. You ought to check out Theatre of Eternal Music. One of my favourite drone groups and they're out of the 1960s. They have John Cale in it among others, if you're a fan of drone it's a must listen.

Drone has existed for a while.

True dat. Ever since the organ was invented people have been holding one chord and seeing all of the textures that they could get. I was just giving a fifties example. Nice piece btw, I'll be checking more of that composer's work out.