The newspaper at least... stops once you've read it -- so that's what an hour (or whatever) gone of your day. That's a major difference, at least.
It blows my mind that my kids will never know what it's like to wait for a bus with nothing else to do.
My point is not that smartphones and social media are great things. But I agree with OP that they are just filling a gap created by something else. It is still possible to be bored or have time solely with your own thoughts with social media available. Why? Because even kids can (and do) understand that social media is not all that great. Of course they have to experience that on their own before they understand it. It's nothing that you can teach them.
bullies don't stop being bullies when you graduate high school, they just change tactics.
There comes a point where adults need to step in, but that isn't immediately because someone decided to pick on someone else. Learning to stand up for yourself is as much of a life skill as anything else.
I moved a lot as a kid and after a while I recognized a pattern. I'd move, someone would decide I was an easy target, I'd get into a fist fight, then everyone would leave me alone.
Someone will be well placed to make your argument - well at least you put your phone away at some point!
The socially challenging bit is the difference not the magnitude I think.
(And, yes, more and more people are putting away the phone because they realize how intrusive it is in terms of time spent doomscrolling, etc.)