These two pages (shown below) are taken from a standard work on general health, so nothing the average person can’t understand. You can easily see which posture is good, which is bad, which is recommended, and which is not.
The first page shows the best way to do various things and maintain spinal health; the second page shows what happens if you don’t take this advice ~ it’s not good news.
Below, the images on the upper page are adapted to show the Cleland riding posture next to the crouched position on a conventional mountain bike.
It doesn’t take much to work out which posture is best for your spine in the longer-term. And these pictures show a static situation, add in the dynamics of riding over rough terrain, and the potential damage can become considerable.
The damage being done to your spine and wrists, even if you ride from a very young age, will not begin to cause you problems until at least your late forties/early fifties. In some it will occur at an earlier age, others, later age.
The posture you use for riding is your own choice.
However, to make a proper choice, you need complete information. For some reason, advice coming from ‘cycling experts’ seems to ignore the advice coming from physiotherapists. You’re unlikely to find this information anywhere else in the cycling world.(Please comment below if it is published elsewhere, I’ve searched for years!)