It's almost like trying to predict if the smartphone leaders in 2006 (Nokia / RIM) would want to adopt this new mobile operating system that hardly anyone uses (android) in 2016.
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.
10 years is relatively short time in hardware, that is only at best 4 cycles. In today's world it is only 3 cycles. The amount of time, energy and resources required to switch from ARM to RISC-V only to save $100M a year makes very little sense. That is assuming you can get the same performance switching over in the first place. Which right now isn't the case in the next 5 years.
And I wouldn't be surprised ARM gives Apple some other patents protection as well as discount on other IPs.
And also from the timeline you can see that the average Apple hardware transition period is 2.75 cycles, so I wouldn't bet too much that their ARM journey will last more than 6 cycles.