Are they actually incapable of learning the right way to do things? Is neuroplasticity playing a role here? Do they not care that their images look terrible when zoomed in on? I guess they grew up in an era when photos frequently had time and date stamps, and if their eyesight is going... I guess that explains both points.
I thought some apps and iOS had already responded to this workflow by doing smart things when users try to create screenshots. If they haven't yet, they should. There's no need in the common case for "screenshot" and "share main image" to have different UI entry points. Just pop up a menu asking the user that they want when they push the button.
No.
Or I can just hit two physical buttons on my phone and grab the image in the form of a screenshot. Because I don't care about the image fidelity, I usually just want to share the thing on Discord or Slack or wherever.
Don't be so arrogant as to assume that people doing things differently than you must be too stupid or old or blind to do it "the right way".