Taking the cramped seat available or paying double or triple for a wider seat is not a realistic choice nor a reasonable one.
Consumers don’t get a check-off list with a product. In the real world life is not granular, and the only way to get granular improvements which cost the seller money is to set a minimum standard that forces the sellers to compete in the market over how much they will raise the price. That helps keep the price increase to a minimum level.
Most passengers are very willing to pay an extra $10 to get an extra two or three inches but they will only have that choice through legislation. The airlines are under no competitive or financial pressure to offer that at a reasonable price. Again, look at the price differential between coach and business class.
Yes, the government is a terrible, inefficient answer to most problems. But it is often the only answer, a least-bad alternative. It’s certainly a better alternative than giving sellers who are in many ways not subject to market pressures free rein with the false claim that the fantasy market will solve all problems.