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 ..

[1] https://configure.ergodox-ez.com/moonlander/layouts/60ynN/la...