The thing is Apple designs its own ARM cores. They don't use the vibrant ecosystem of ARM for all the important parts of the CPU; they compete with it on their own and manage to win. They only pay a small amount per device for an architecture licence that's very generous for Apple, in large part because Apple was an initial seed investor and first customer of ARM.
If all the current investments in ARM don't suffice to beat Apple's own efforts, I don't see how they could ever need to use RiscV's community to achieve their goals.
As long as ARM keeps the architecture up-to-date and the fees minimal, Apple's staying.