For post processing it is important to have a high resolution. This has
nothing to do with the final result which of course has to be smaller
to fit the screen in the end.

By the way, screens do not have a fixed resolution of 72dpi. Most
screens nowadays will be running near 100dpi but this depends on the
resolution set by the user. The term dpi only really applies to
printing.

I am quite sure that National Geographic prints at a higher resolution
than 300dpi (my printer at home does 600dpi).

I still recommend not to use any kind of scanner demoiré features
because scanner software usually is not as good as Photoshop. Each
post processing step takes information from the original leaving less
options for the guy putting it all together in the end.