To expand and agree: turnips, beets, hard whole grains, if their baby teeth aren't visibly worn by the time they fall out, it wasn't hard enough.
Jaw and face bone grows by stimulation. It's not just a dental thing - it's sleep apnea, sinus infections, facial structure, voice timbre, and attractiveness.
If it's enough, they won't even need their wisdom teeth pulled - having your wisdom teeth pulled is substantially a standard american diet issue, not a human genetic issue.
It's funny because I mildly disagree with your core premise (orthodontics are unnecessary), they should just be necessary as a disability accommodation, essentially. If we had 10-30% of the population in wheelchairs because we didn't let kids walk I would find the wheelchair industry odious as well.
Right, most of the orthodontics websites say 70% of people need them. I understand wanting to justify a total market size but if people actually believe that, it's getting out of hand.