ARM itself has little to no impact on open source community. They only license chip IP, they don't make any chips themselves. And most of the ARM architecture is documented and open, with the exception of things like the MALI GPU.

Whether or not some SoC (e.g. in a phone) is going to be supported by Linux doesn't depend on ARM but on the manufacturer of the given chip. That won't change in any way.

ARM maintains the GCC toolchain for the ARM architecture but that is unlikely to go anywhere (and even if it did, it is open source and anyone else can take it over).

The much bigger problem is that Nvidia could now start putting squeeze on chip makers who license the ARM IP for their own business reasons - Nvidia makes its own ARM-based ICs (e.g. the Jetson, Tegra) and it is hard to imagine that they will not try to use their position to stiffle the competition (e.g. from Qualcomm or Samsung).