If Nvidia don't produce a decent chip, I can guarantee that Nintendo will go elsewhere. Potentially there may be an Nvidia laptop chip they can switch to, but that wouldn't be a direct Tegra replacement either.
Mobile chips are mostly compatible with each other (it's all just Vulkan toolkit, there isn't really an Nvidia-only toolset these days). Nintendo could look to Samsung or maybe a custom ARM Mali GPU - those guys have the mass market to do R&D. Nvidia never built a marketshare in that space - I always assumed that they just gave Nintendo a good deal to cut their losses on Tegra...
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.