Actually that's a possibility ,but a very limited one and limited to the point of view of Game developer, which in their engine tend to use bump(rare) or normal or displacement(very rare), not all of them at once.
Also it will not help definitively in the workflow I mentioned above (expecially if working with photogrammetry), using 3 maps at the same time gives maximum flexibility on large scale, medium scale, micro scale features, while keeping the polycount in the right compromise for 3d rendering applications (maya, 3dsmax, modo, c4d, lw...) (not game engines).
That's why it would be great to paint all of them at once. In the beginning it could simply add an extra layer to paint all of them at once.
But it would definitely need a subdivision surface algorithm (opensubdiv/catmull clark) integration in the PBR painting mode later on.
This could slowdown the viewport so it should not be on by default but enabled at press of a button (both displacement and subdivisions), and selectable like the three top buttons.