Did you reply to the wrong person? The person you're replying to said nothing about buying expensive chairs.
These are not explicitly stated in the comments, but can be reasonably inferred from the context.
I sit in many weird positions throughout the day.. lean back, feet on a box. straight up and down, legs crossed, lotus, and yes, I squat on it too.. sometimes on my knees, sometimes the chair reversed.. pretty much anything that keeps my head above the table.. totally privileged, I know :)
Hell, the last chair was thrown out after it literally broke apart, it and before it did, it was crooked to one side for a few years. I'm 35 now, my back is perfectly fine. It's not about the stuff you've got, it's about you.