There are still plenty of people doing short rides, but taking multiple weeks or months off to bike across the country or abroad has become nearly impossible for today's generation.
To be clear it's not the expense of the trip itself. Bike touring is cheap. It's the lost wage due to not working. This isn't Europe where even minimum wage workers have 6 weeks guaranteed paid vacation time.
I don't think this has changed as much as you think. In my perception there have always been barriers to people taking off in prime working ages for an extended tour. More barriers than those either at start of career (e g. gap year) or at end (e g. retirement). Some of those are very real in terms of $$ but there are also family, career considerations and sometimes an extended tour isn't a priority compared to everything else.
Twenty years ago I spent eight months cycling around Australia. I saw very different nationalities depending on where I went (and not just for cycling):
- Americans doing a "three weeks to see Australia with stops in Sydney, Melbourne, Uluru, Great Barrier Reef and one more stop"
- Japanese and others spending 36 hours to visit Uluru with one sunrise and one sunset
- Australian "grey nomads" pulling caravans for multiple weeks to cross the north during the winter
- German speaking folks renting Britz camper vans to travel through outback areas
- Etc...
Nobody expected me to be American (they guessed Canadian more often) because I was spending too much time on my holiday...
Not sure it has changed as much as you think in the last twenty years...