Most tall stools I've seen have something at about the right height to rest your feet on, whether just a brace between the legs of the stool or a ring that looks like it's intended as a footrest.
Even without that, a detached footrest would still be rather easy to find.
"Because then you would be forced to sit with your legs dangling"
Most stools let you adjust the height without problems, so you could put your feet on the floor. Some of them you could even attach to the floor, so you could use it as a support.
You'd probably want a stepstool or applebox anyway when you're standing, just to vary your feet positions. That could be used as a footrest when sitting.