Re:
Up until the mid 90s and the advent of fairly long travel suspension and suspension corrected frame geometry, ATB/MTB frame geometry wasn't that far removed from a road/touring bike and as such, slap a set of road tyres and it made a robust and capable touring or commuter bike.
I think modern MTBs are much more wholly off-road focused and no longer make a passable road bike with just a tyre change.
This may be in part due to the fact that more people now will drive to the start of a known route and do an entirely off-road ride. Whereas I used to, and still do, set of from home on the bike and go out and explore, which would often involve a fair amount of riding on tarmac.
I wonder if the age that people start riding has also changed. Reading posts on the forum, many of us started riding as teenagers, before we could drive, so driving to a ride wasn't an option. When I was a teenager, most kids had a bicycle, even if they weren't "into cycling" specifically, they would occasionally use one for getting about on, or perhaps riding to school on.
The number of cars blocking the roads around schools in the morning and evening would suggest that children these days are ferried around in cars far more than they used to be. When I was at both primary and secondary school, only a handful of children were driven to and from school, the vast majority of us walked, cycled or got the bus. I imagine that far fewer children have a bicycle these days than used to be the case, so perhaps those that start cycling do so later.