yes, I have a 2.2Mp file from a Fuji camera; the sensor was a diamond pattern, and the camera itself interpolates to 4Mp. I hav a print that is blown up to 11x14. it is somewhat grainy on close inspection, but otherwise quite good. it’s slightly better than 100dpi.
Depending on viewing distance, about 75dpi to 100dpi is as bad as you want to get. (However, if you look at photo posters, even commercial ones, you will see the quality can be quite fuzzy - we’re just meant to look at them from farther away. )
300dpi is an arbitrary but doable number because the magazine has to set a minimum quality. You don’t want fuzzy pictures in a quality magazine. part of National Geographic’s appeal is that the photos are not only striking, but usually highly detailed - which comes from the resolution of the photos themseves. it also depends on detail. Fuzziness in a flat expanse like a face is less noticeable than in a detail like hiar or clothing weave.
I know that even with the 2.2Mp Fuji, when I first had photos done at 4x6 the phenomenon I decribed was “painfully sharp”. being used to crappier point-and-shoot prefocused film cameras, the resolution quailty of digital was overwhelming. I did a photo book in My Publisher at 11x15 of these pictures for a trip I did to Italy, and some at full page size still look good.