WinterSnowfall: Your "facts" are an intrinsically subjective method for the classification of music, which is ever changing.
Ironic, considering you are the one to use the genre and sub-genre defining names like "doom metal", despite claiming it's highly subjective thing to do.
WinterSnowfall: Sub-genres slowly take form and evolve into genres that eventually spring sub-genres of their own.
Wrong. Sub-genres do not "evolve". The musical genre evolves. During that evolution happening, the new sub-genres are being born, and that is why I listed all of them in my original comment. Sub-genres do tend to blend here and there, but it goes to create new sub-genres, not change the already existing ones.
Horror movies are Horror movies, it doesn't matter the year they have been released. Thrillers are thrillers, no matter which decade they were recorded.
WinterSnowfall: Leave the absolutes in the realms of mathematics and logic.
Going with that kind of "logic" one could make everything relative by saying "There are no music genres and sub-genres, there is only music." Which is wrong, of course.
Here is why genres and sub-genres exist.
Person A: I like Nightwish. Can you recommend me more similar bands?
Person B: You might like Therion.
Person A: Thanks. They are good. Any band else?
Person B: You might try out Haggard.
Person A: Wow, I am falling in love with these type of bands. What is their musical direction?
Person B: Heavy Metal music, done in Symphonic Metal style.
Person A: Nice, I will check out some more Symphonic Metal myself.
If we didn't have accurate sub-genres, conversations would have been this awkward:
Person A: I like Nightwish. Can you recommend me more similar bands?
Person B: Try out HammerFall.
Person A: That doesn't sound like Nightwish.
Person B: It's all "metal".
An extreme form of relativism example:
Person A: I like Nightwish. Can you recommend me more similar bands?
Relativist: Eminem.
Person A: What?
Relativist: It's all music, you bigot.