How are anybody's teeth at 40 if they haven't visited a dentist? Brushing is a cause of receding gums, gum infection, tooth loss, and deep pockets around the gums. Modern sticky flours and sugars also do a number on your teeth, giving bacteria a wonderful place to live.
That being said, if you don't brush your teeth, food will find places in your mouth to sit and rot. Brush your teeth for everyone around you, not for your teeth.
For me they made an excellent necklace (and an amusing way to disturb my sister), but in bygone generations they with the emergency backup replacement teeth because so many of your teeth fell out.
I even saw something about cavities in dolphins... and, if I remember right, the thing about humans not getting cavities before the advent of the modern diet stems from a dentist who had some unorthodox theories about dental care that were unsupported by systematic research.
Oh, it's the Weston Price foundation people who make that claim...
https://rationalwiki.org/wiki/Weston_A._Price_Foundation
https://sciencebasedmedicine.org/sbm-weston-prices-appalling...