That is not an argument for more non-modifier keys; it is an argument for more modifiers. Why? Each added non-modifier key adds X more modifier+non-modifier keybindings (e.g. C-n) and X-1 more modifier+modifier+non-modifier keybindings (e.g. M-C-n), where X is the number of modifiers. In practical terms, a keyboard with control, alt, shift and super only gets four two-key and three three-key bindings with each additional key. That is not great.

Adding a modifier, OTOH, add Y modifier+non-modifier and Y modifier+modifier+non-modifier bindings, where Y is the number of non-modifiers. So adding a hyper key means adding , say 104 additional keys (on a keyboard with 52 non-modifier keys). Adding a compose key means adding … another 104 additional keys.

My current keyboard has control, alt, super, hyper, compose and shift. A lot of folks add raise & lower, for even more modifiers.