"My appeal is to software and hardware developers to ensure that a 12 point font will be rendered the same size everywhere, regardless of screen size and density."

This is a not very well reasoned appeal. If it were implemented, every Keynote presentation would be unreadably small when projected onto the big screen, literally the same size as the presenter holding up a piece of paper for the audience to read with their binoculars. Likewise, no more than a few characters at a time would be visible on a phone.

There is simply no serious counteragument; nobody can say making things unviewable would be a step forward.

The best you can do is complain that the name of the units is misleading. Fine. So what about introducing a new unit that is truly faithful to a real world measuring stick? You can argue for that all you want, but guess what? Nobody in their right mind would actually use it, outside of exotic scenarios, because generally you want to avoid the above effects. You want your user interface to be visible to users at different screen sizes meant to be viewed from different distances.

Thus, even if we were to introduce a new "TruePoint" or "TrueInch" unit, it would be largely.. and I beg you to excuse the pun here.. pointless.