The 20nm fabricated tegra chip may not even be possible soon. Fabrication doesn't stand still. Many revisions are practically forced on the manufacturer because the main chipset has to be updated to the newer fabrication. So there will be an internal revision at the very least unless they build too many and end up stockpiling them like the wii u but this seems unlikely with the Switch level of demand at the moment.
Updating home consoles is not the same as portables. It makes more sense to update portables in many ways because newer fabrication actually improves the product by extending battery life, lowering weight etc. Often Nintendo does a slightly crap first portable and then introduces much improved versions later and this is likely the case with the Switch. Who doesn't want a Switch with better battery life, more compact and at a cheaper price?