Would they even need to jump to something else? Apple's implementation of the Armv8-A ISC is unique to their hardware so any reason they could not just continue to develop it independently should NVidia launch a new ISC?
Because being on an instruction set that just you use is more expensive than being on a "shared R&D" expensive solution. Apple doesn't need a whole different set.
So if RISC-V was a larger, healthy ecosystem 8 years from now with millions-billions in shared R&D being poured into it by most of the major players it could make sense to transition to that. It is the shared R&D costs of ARM that make it attractive to ARM. The same way OpenGL used to work for Apple . The unix terminal system works (the open source base they can lean on) , etc.
If Nvidia drivers ARM into the crapper than Apple can get off. And Apple won't be the only ones. There are lots of others who can gradually get off also. If the paying herd goes somewhere else then so can Apple. None of the other options are really have the herd moving that way, but that can change over time. Once big players got onto the Linux bandwagon the "enterprise" Unix options died off substantially.
Apple have jumped offf of 68K , PPC, and x86-64. One more in 10 years won't hurt if there are financial upsides to it. Apple doesn't slavishly cling to backward compatibility. that makes the instruction set long term disposable as an option.
Apple has never run off and used an instruction set that nobody else was using. Small tweaks in some extremely narrow corner cases isn't all that unique of an instruction set. Shared eductional/training exposure on broadly used instruction sets , tool-chain development ( optimizations , tuned libraries , etc). If all that dramatically dwindles over a decade or so for ARM it would be time to consider going with the herd.
Apple doesn't have to switch any time soon or even several years. Nvidia doesn't "have to" drive ARM into the ground. The current inertia on ARM is high enough for many players that Nvidia could milk as a cash cow for quite a long time. For which Apple could just kick the can.