I was in my mid-20s when Nirvana was huge...here's my take.
Rock was troubled. There was no important rock band anymore. Guns and Roses had petered out...the 60s monsters were in their 50s and therefore too old to inspire us kids, the 80s hair bands were all gone or not important anymore. REM was doing 'Losing My Religion' quiet music.
It had been a long time since any significant mainstream guitar music had happened. We had all grown up on Journey and Foreigner and whatever...guitar music the effortlessly turned out catchy, important music song after song. Then that stopped.
Until Nirvana came in and blew off the roof. The band had it all. A new, vivid sound, attitude, style, and they could write song after song, interesting, edgy and catchy. It was like a reprieve for white rock and roll. It felt like rock was not dying.
But then Kurt did die, and rock with it. The last rock band that mattered.
Rock and Roll
No rest, no peace, keep rocking.
1954-1995
That's why Nirvana is important and not overrated.