They aren’t going for detachable controllers, most likely. Theoretically they might make a future screen + SoC modular so you can swap a frame (it would kinda make sense from an engineering perspective to build the center similar to a tablet IF you can hit all required specs that way), but even for Valve that’s unlikely. They may prototype it but likely won’t release it.

Would OTOH be nice with a Steam Controller 2 which can fold flat to fit in the same case as the Steam Deck. Also not likely to happen 🤷