You could chew on your fingernails...
https://www.ox.ac.uk/news/2014-01-07-starchy-food-led-rotten...
But I think the more common prehistoric tooth problem was severe wear from using the teeth to process things like fibers and hides.
I think it was fairly rare in Europe, but IDK how well those numbers capture what was common for the majority of the human population living elsewhere.
The first English king to be definitely alive on their 70th birthday (though no longer "in office") was Philip of Spain (jure uxoris) in 1597, so not a medieval king. That is Early Modern Age.
Elizabeth I. didn't make it, though barely, and so the next to reach 70 was George II. in November 1753! Only since the second half of the 18th century is it common for British monarchs to reach their seventies.
Richard Cromwell lived to be 85, but he was never a king, only Lord Protector.
Edgar Aetheling lived to be 73, but he was never king either, due to certain William arriving en force from Normandy.
Assassination definitely brings down the average. But a fair number of English monarchs managed to die in bed. (I was gonna write British, but no: the Scottish kings practically never died in bed. Unless they were stabbed in their sleep.)
As for the common folk, if you look at actual medieval cemeteries that were excavated and studied, the peasants didn't live long either. The age of death can be assessed by looking at the bones, and already the above 50 cohort is somewhat thin, while the above 60 is infrequent.
You underestimate the effects of hunger on mortality. Prior to introduction of potatoes (e.g., ~ 18th century in much of Europe), failed crops would be a common occurrence, happening ~5-6 times during life of a normal rural person. If two of them happened back-to-back, the resulting mortality was already serious, and older people would often be victims. It made sense to use whatever food was left for the younger, stronger generation which was still able to work.
Famine was basically never a concern for the royalty. We have a record of the English king going dinner-less once, but that is not a threat to your life.
BTW If you really want to find a relatively long-lived sector of the society, it would be the high clergy, which had all the upsides of noble life (food, warmth in winter) and almost none of the downsides (most wouldn't fight, murder was less common). This is the only "job" which saw some 70 y.o.s still alive and active, mostly as cardinals.
I did not dispute that this was likely rare in medieval Europe (for the same reason you cite).
This is absolutely untrue. What is your reference?
Never in history have so many people had such "good" looking teeth, but they involve an enormous amount of prosthesis and amalgam. Veneers aren't good teeth, they're intentionally destroyed teeth used to root false teeth.
And brushing, although it keeps teeth clean and not stinky, deepens the gum pockets around teeth that host the microorganisms that will eventually uproot them.
Check out Nutrition and physical degeneration book by Weston Price.
All you need to do is to look at the pictures in the book, you don't even need to read it.
What the article doesn't mention is the salivation that Neem twigs cause! Neem trees also produce a biocide called azadirachtin and although the concentration is low in twigs maybe it helps clean the teeth when used a lot?