Nope, I would strongly disagree — kids don't roam anymore and screens fill the time at home, but the social fabric of the US has been collapsing long before smartphones and arguably even the popularity of the internet. Devices have perhaps accelerated the problem, but they're far from the cause.

The dual-income household expectation and collapsing middle-class (lost share of productivity) would be the most to blame from my perspective... there's also a major daycare shortage because of this, the lifestyle changed and surrounding factors haven't caught up. There are kids regularly staying at after school programs until 6pm because parents are working and no one wants kids wandering around on their own, even in teen years. My local mall has banned kids under 18 unsupervised as yet another example.

I'd say the main reason for this is how much more complex life has become. In the past basic life for a normal person was quite simple. Now it requires much more from a person to handle the basic necessities in life.