I'm pretty sure parent wasn't talking about Nintendo's software tooling, they were talking about the machinery to assemble the hardware. From that angle, I suspect that Nintendo would not be able to simply order something off the shelf that would work with their existing assembly lines. There is no "just" when it comes to changing hardware in a system like the Switch--which is part of why you don't see a bunch of aftermarket upgrade kits.
Related, I invite you to review the comments and note how many are focused on the battery life of the Deck, and specifically on how short it is. Even if Nintendo could just upgrade to a faster CPU, that would bring increased power draw, which would lead to either shorter battery life or needing a bigger battery, at which point you are either at people complaining about your battery life or once again back to industrial design and retooling your assembly line.
Tldr: They can't just bump the price by $200, drop in a Threadripper, and call it a day.