Much of the shrinking market is due to the fact that laptops used to need to be replaced every three or four years, and now only need to be replaced every six years or so. That's about how long it takes for the hardware to wear out, and for the next generation of hardware to be significantly better.

In 2022 Intel plans to have 7nm CPUs available. Those laptops will be considerably more powerful, and laptop's purchased today will be starting to fall apart. So it's unlikely that folks are going to go ten years between new laptops any time soon, meaning the size of the market should level off somewhat.

There is plenty of room for Apple to continue to make improvements within this general form factor, and to reduce the price. And if tablets or phones really do somehow make laptops obsolete, then Apple will just license or open source OS X. That way it wouldn't have to deal with making laptops, but developers would still have a good platform for making iOS apps.

The idea that OS X or MBPs are going anyway makes no sense, at least for the next decade or so.