More than that, a second implementation of CUDA acts as a disincentive for NVIDIA to make breaking changes to it, since it would reduce any incentive for software developers to follow those changes, as it reduces the value of their software by eliminating hardware choice for end-users (which in some case like large companies are also the developers themselves).
At the same time, open source projects can be pretty nimble in chasing things like changing APIs, potentially frustrating the effectiveness of API pivoting by NVIDIA in a second way.