I don't think it'll be close to that long. Rumours are that they'll release an 8+4 core this year, which is enough for everything bar the top-end iMac Pro and Mac Pros, and I'm sure they can get out a 16+N core by 2021 or early 2022. That would be as fast as any Intel CPU on those models. And heck, the margins on those models are so large (they sell a $3000 Xeon W for $7000) that they could comfortably push past yield issues to throw out a larger die than that.
Good point. I still think 5 years is a realistic time-horizon for how long before they start phasing Intel stuff out, but that could just be like that Bill Gates quote about how people tend to way overestimate how much progress will be made a year from now and way underestimate how much things will change 10 years from now.
