Main downside is that having swappable components adds size and cost, which is why laptops are so much less modular than full size PCs. For something like the Deck, which is trying to be as small and cheap as possible, I doubt we’ll see anything modular for a long time.

Valve could possibly sell upgraded motherboards that you could use with your original screen/etc. However before ifixit sold deck parts, there was a leak of the upcoming parts and prices. At the time, replacement motherboards were planned to be sold, but they planned to sell the motherboard for $350 (when the cheapest deck was $400). Ultimately they ended up never selling the motherboard, which makes sense when considering how expensive it was compared to the overall price of the unit.