When and why did the 20th century bad boy aesthetic (with motorcycles, etc) stop being cool?

[Agonized-Statism]

That's a pretty sweeping statement. Long hair has come and gone over the years (mostly gone), the negative effects of cigarettes became known (but theoretically safer vapes have been taking their place), motorcycles have never been a practical means of transportation for most people and biker gangs never recovered from their leaders getting busted for drug distribution and murder during the 'tough on crime' days, and rock exhausted its experimentalism- becoming a tired mainstream sound like jazz before it- and stayed a largely white niche. New generations like the wider variety of sounds you can get out of electronic music at a lower cost and with fewer barriers to entry.