You have to balance every cost against your targeted retail price, though. Adding a few extra dollars per APU would increase the cost, and could be balanced out by trimming a few mah from the battery capacity, but I assume that nintendo found the sweet spot in their eyes. Also, cost per transistor is fairly flat. So you wouldn't be saving money by going smaller at all like you could in the past, it will only be more expensive.A more advanced node on a portable would give them both better performance (and they are trying to get third parties on board, so every MHz is important) and better battery, which means people playing your platform for longer, using your store more and using your services. Is saving like 10 dollars really worth it?
The economics of scale play a big part here. Even if the system is a failure and only sells 10 million units, that would add $100 million in costs if making a change added $10 per unit. And if it's successful, and sells 100 million units, that's $1 billion in added costs.