Music evolves, that's why rock from the 1950s, 1960s, 1970s, and so on, sounds distinct as time progresses. That's why trap artist and Grandmaster Flash occupy the same genre, yet sound completely different. When I hear people complain, the get off my lawn vibes are strong.
...And as music evolves, new sub-genres emerge so that people can more accurately describe what something is.
For example, instead of calling anything hardcore "heavy metal", we have such sub-genres as:
*Progressive Metal, which is marked by such things as complex subject matter and complex time structure (examples: Crimson Glory, Dream Theater, Fates Warning)
*Power Metal, which is marked by a hardcore sound brought about by maxing out one's instruments (ur example: Dragonforce)
*Death Metal, which is pretty much what it sounds like (no, I don't listen to this)
*Christian Metal, which is also pretty much what it sounds like (ur example: Stryper)
*Christian Death Metal, which I have in fact been assured is a thing
*Hair Metal, which is the generic term for bands who combined 1970s glam stylings with a more commercialized version of heavy metal (examples: Twisted Sister, Poison, Cinderella)
*J-Metal, which is generally employed as a catch-all for all Japanese heavy metal bands but in
specific refers to those bands that are marked by a particularly unique sound that blurs the lines between genres and the Japanese singers usually being fluently bilingual, allowing them to switch between singing in English or Japanese depending upon the audience. (ur example: Loudness, with a caveat about the infamous Mike Vescera era^)
Et cetra...
^When Loudness' original lead singer left the band, the band's international distributor forced the group to accept an American lead singer, Mike Vescera, in the belief that Vescera's presence would make them more marketable to non-Japanese audiences.
While Vescera was a good singer and the "Soldier of Fortune" album is musically in keeping with their previous releases, he was ultimately a poor fit for the band and cost them a *lot* of their hometown support.
His tenure in the band is divisive even to this day, and for all intents and purposes everything made between the time the original lead singer left and the time he returned has effectively fallen down the memory hole despite still being readily available through iTunes and other online retailers.