None of us knows what's motivating him."Reaching a massive customer base" isn't necessarily a primary motivation for innovative product people.
A company the size of Apple, that's as established as it is, has many institutional handcuffs that one may want to be free to work without.
The scope of "what's possible to make" at Apple is fairly narrow actually.
What I do know is that when you get to that age - retirement is something you start thinking about. How do I know that, been there, and so has been everybody I know in that age group