The "kids are getting less active these days" narrative has been happening for decades now, I can at least say it started in the 80s-90s.
I appreciated OP's nod to blaming parents. In the 90s boomers would treat it like it was a kid problem, and not THEIR problem even though they were the one raising the kids. "They just sit inside and play video games!" well because you allow them to.
Once upon a time, kids could play outside from sun up to sun down. Then the 80s came and with it video games, more TV...and...America's Most Wanted.
Look I appreciate John Walsh for what he went through. But he put the fear of god into a generation of parents convincing them that their kid will be the next kidnap-murder victim, something that was truly a rare occurrence. And suddenly parents were freaked out, locking their kids inside, having to know where they were at all times, and then it was no longer, see you for dinner, it was organized sports or stay inside. And slowly, more gadgets and technology was there to entertain us, and yada yada yada, post COVID here we are.
Which brings me to the above post. I like this post because I think it's the primary problem. For too long we have relied on organized sports for kids to get exercise, instead of just regular play. But organized sports have other goals in mind: to find the next Lebron, the next Mahomes, the next generation of elite athletes. They are intense, competitive, expensive, and they weed average kids out at alarming rates. So because there is no other outlet for the regular kid who needs the exercise, they just spend their time sitting at home.
I hated organized sports as a kid. I was short and uncoordinated and I was bullied a lot by coaches and other kids. It was only when I found running that I enjoyed sports, but the idea of that being the only opportunity for exercise sucked for me, and for a while I was headed in the direction fo couch potato.
I think the answer lies in how parents choose to spend time with their kids. Don't just force them outside and sit and watch TV while they play. Do things with them, go on hikes and bike rides together, go roller blading or play basketball with them. Make it a family activity not a chore. Plan a weekend together--"we're going hiking...here." So they plan for that, so you aren't interrupting Zelda to push them outside with no plan.
Organized sports in this country need to become a lot more hospitable to the average kid. There needs to be more levels, more leagues and make things more affordable. Kids need to decide for themselves how seriously they want to take things.