For a querty or qwerz layout that would indeed work, if you moved one symbol key (- for german layout, ? for american).
However in my keyboard loayout there actually is a letter in that place (j) [1]. And while I'm willing to compromise on modifiers, page up/down and the like, I draw a line at the stuff that I spent a lot of effort on learning to touch-type :)
As for using a navigation layer - perfectly happy with that 90% of the time, I already use that today, but for some things I just prefer actual cursor keys.
For example selecting an entire word in an editor, you press shift to select, ctrl to jump a whole word and cursor for the direction to jump in. If you add a modifier to that, you start breaking fingers ..
Ah yes, for that layout with 4 extra keys you would have to compromise. E.g. move the ',' or '.'to be shared with the right shift key so that you can have the arrows keys in normal layout. Or just have the left and right on the base layer and all 4 in another layer.
As for using a navigation layer - perfectly happy with that 90% of the time, I already use that today, but for some things I just prefer actual cursor keys. For example selecting an entire word in an editor, you press shift to select, ctrl to jump a whole word and cursor for the direction to jump in. If you add a modifier to that, you start breaking fingers ..
[1] https://configure.ergodox-ez.com/moonlander/layouts/60ynN/la...