The hidden costs are in the ER visits

My parents dug a hole in the ground for a trampoline, so it was flush with the ground. It was great because you couldn't accidentally fall off, and it was safer than doing any given stunt on regular ground.

Or your home insurance company dropping your coverage because you didn't declare the trampoline yet they found it while surveying aerial imagery

Almost everyone I knew who broke a bone growing up broke them on a trampoline. As an adult, every story I've heard involving someone's kids and a trampoline ended up involving a cast at best.

... And yet. I wonder. I can remember the feeling of freedom and joy in my body as a child, joy in movement itself that I have never recaptured in any meaningful way no matter how I ever tried to motivate myself to exercise...

Swimming is also dangerous, but is considered a very healthy and useful activity and skill. I could see a similar cost/benefit analysis for trampolines and other gymnastic equipment.