Alternative radio in the 90s got those consensus audiences because it was the best/only option for hearing new music.
At the same time, the music attracted a consensus community. Especially in big cities. They got together and listened to music in large groups, and built relationships through the music. It was almost a repeat of the peace & love experience in the 60s, but with different issues and different music. That doesn't happen in alternative music now the way it did 25 years ago.
They aren't growing up with radio as their main music source, partly because radio is obsessed with 25-54 and failed to invest in serving those younger listeners when it mattered.
There's a lot of mythology in that sentence. People do lots of things and use lots of products that were not part of their experience when they were kids. The radio stations boomers grew up with didn't target them when they were teens. Radio stations aren't there to be a "main music source." That's simply a perversion of what radio is.
You're making a music problem a radio problem. The music industry has ceased to invest in music for young people. The only company that used to do this was Disney, and now they're also obsessed with 25-54. So who will invest in the music for young people now? The young people themselves. And that's bad news for those who seek to create consensus music. It just furthers the musical cocoon.
Country is a superstar format. The same relatively small bunch of artists stay on the charts for years churning out record after record. Nashville is a well-oiled, tightly controlled machine that creates a product that still works well on radio and satisfies an audience that craves formula and familiarity, not new music discovery.
Part of that isn't true. The country music industry is focused around launching new artists. There's a country radio convention going on right now that's focused on introducing dozens of new artists to radio programmers in order to get their music played on the radio. That's what's missing in the alternative format. There is no investment on the part of alternative music as a genre to do what you say Nashville does. If there was, we wouldn't have the problem we have right now. The problem with alternative radio begins with alternative music. Great music makes for great radio. In alternative, everyone has their own opinion about what great music is. Radio stations can't make money with that.
You can't see music radio as something that's separate from the music industry. They have to be partners and work together, or it all falls apart.