You know successor systems are worked on not long after a system launches right? Now that Switch is mid life. Likely already working with Nvidia on the next chip. Doesn't have to be tegra. Tegra is done. What they will very likely do and are working on right now is a custom chip and the next version of NVN api.
No it doesn't have to be Tegra, but by extension it then doesn't have to be Nvidia either. I just don't see that Nvidia are putting in significant R&D for that type of chip (compare AMD who basically live to build both the Xbox and PS). Maybe there will be a minor upgrade to a Tegra++ or something.
If Nintendo are relying on Nvidia, they have backed the wrong horse. Well... unless the next Switch uses a desktop Nvidia GPU :)