And what they did sounds nothing like anything the 70's called "disco." Not unless you're willing to call the more hardcore synth-based New Wave stuff like Depeche Mode, Ultravox, or Television "disco."
To kill off metal, I see two possibilities:
1 Suicidal Tendencies' major breakout album and single Institutionalized gets a the mainstream contract that OTL went to Guns and Roses for Appetite for Destruction, and then, unlike Gn'R, they and their fellow Hardcore Punks actually follow it through with subsequent albums just as loud and driven as before. With the 3:00+ guitar solo suddenly passé again, there's no room for the likes of Metallica and Pantera to break out into the mainstream. Music in the mainstream is that much harder than OTL, but Metal has for all intents and purposes been squeezed out.
2. In 1991, Slint's "Good Morning. Captain," from their album Spiderland, becomes "The Song That Killed Metal." Slint stays together another three or four years, and as none of their members died from self-destructive behaviors, they (along with the rest of the Math Rock Scene) continue to evolve Math Rock into Post-Rock. To someone from the late-70's, popular mainstream music is that much more alien and unfathomable.