Chair seats happen bc you need to work on your should-hip-heel lineup, and shouldn't be affected by the width of your horse...Seriously, THIS is the natural riding position..."
A lot depends on the rider, horse, and saddle. The natural riding position is one that doesn't fight gravity. That means your butt should be in the lowest part of the saddle, and your stirrup straps vertical.
With my western saddle, that WILL mean a chair seat. With my jump saddle, it means a mild chair seat - although it is my favorite saddle for 2 point. With my Aussie-style saddles, it also puts me in a mild chair seat - about heel-belt buckle alignment. I don't own a dressage saddle, but I would like to try one someday to see if it puts me in a hip-heel alignment. I'm guessing it would - on a narrow horse.
When I took up riding at 50, I always had a chair seat. 40 years of daily jogging gave me such tight hips that no way - no how would my heels ever align with my hip. 4 years of regular riding has loosened me up some.
But on a wide horse, with MY body, trying for hip-heel alignment would force me to squeeze with my knees. I do enough of that already! And that is, IMHO, a much worse fault than a chair seat.
The only horse I've ridden bareback put me in a chair seat as well. Based on ancient Greek art, I don't think I"m the only one...