The steam deck touchpad thing is because it needs the steam input drivers and software.
Realistically if valve decoupled the steam input driver and software from the steam client then you could use it with out the client.
But that's like bitching you can't use a video card with out a driver... It's kind of a well duh
Even then so long as there's some level of touchpad support you should still have basically functionality so long as there detected which the deck has.