The problem is that IMO things are slowly getting to the point where it's a medical problem, especially for older people. And the median age of people in the US is slowly ticking upwards. Consider things like Deep-Vein Thrombosis for longer flights: https://www.cdc.gov/ncbddd/dvt/travel.html.

"Some airlines suggest pulling each knee up toward the chest and holding it there with your hands on your lower leg for 15 seconds, and repeat up to 10 times."

Most modern economy class seats have absolutely no room to do even these limited sorts of movements. And don't even get me started on how bad they are with someone with a disability. Plus, as someone else mentioned, some employers, etc. make you take the lowest cost flight as a matter of policy.

I think in a way this is like healthcare (the analogy is loose, I admit). Sure, some people, maybe even a majority of people can do fine with the unregulated version. But if it is actively dangerous to a smaller segment of the population, regulation will help bring it more in line with normalcy for that smaller segment.