Kids have a lot more things to occupy their time.
the internet - youtube, social media
tv programming- most households have one or 200 channels of programming
cell phones text messaging
Its no wonder kids don't have the time or the interest in model building.
I believe model building will survive as a hobby but it is going to have to evolve from how most of us think of the hobby. As stated 3d printing is just starting to have an impact and will have a major impact on the hobby. Plastic model kits have only been around since the 60s. Before that most building was done with wood.
Technology has had a major impact on our lives. Some of it is good and some bad. Regardless it will continue to shape what we do with our free time.