The exact quote:
It is important to us, and we’ve tried to be really clear, we are not doing the yearly cadence. We’re not going to do a bump every year. There’s no reason to do that. And, honestly, from our perspective, that’s kind of not really fair to your customers to come out with something so soon that’s only incrementally better. So we really do want to wait for a generational leap in compute without sacrificing battery life before we ship the real second generation of Steam Deck. But it is something that we’re excited about and we’re working on.
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.
Regarding size, the steamdeck already has an M.2 for storage, and a CAMM2 module for ram would take about the same amount of space as a second M.2 drive. The only other major thing for repairability/upgradability would be less glue on the battery and threaded inserts, which doesn’t add size.