Correct, you can’t simply load a .pln while flying in MSFS. Afaik you could so so in FSX/P3D though. Asobo simply needs to implement that function again.
Totally OT, but you can actually load a plane while in flight. It only takes 4 or 5 more clicks than it used to in FSX/P3D but you can do it easily:
To go even more OT: you can also transport your plane to another airport/location using the appropriate option in that same dev mode menu. Again, no need to go all the way back for the menu for this!