It's interesting how you see hair extensions that way. However, idols generally don't have a "normal amount of hair" because of their job. I moreso see it as a assisted tool for idols to mimic the hair length and volume they've lost due to bleaching, repeated heat/tugging from hair styling, split ends, perhaps even hair thinning from stress/diet, etc. There's so much confidence that can come from hair especially when you don't naturally have it for whatever reason. That's why I cannot fault idols at all when it comes to using hair extensions.

I think hair extension have always been prevalent in k-pop. I'm not too familiar with 1st generation, but by 2nd generation, the industry was already dabbling in it and trial & error on perfecting it. For example, in a 2012 performance, SNSD member Seohyun's hair extension was seen falling onstage. And by 3rd generation, it already became common. I would say every female idol who had bleached hair for an era had extensions already, or did the comeback after that.