It will happen just with any other market before, after reaching a certain plateau, only a niche will care about.
How many people do actually tune their cars, specially modern ones that require all sorts of on-bord computers?
Or customize their VCRs, TVs and so on?
PC have become what every other home computer system already was, plain appliances.
Before the PC all other home computer systems had all their OS, or at least part of it in ROM and where mostly only expandable via external devices on their connection port, very few models had internal expansion bays.
The market has come to realize that the PC flexibilty doesn't pay any more in the age of "good enough hardware" and razor thing margins, so back to the old appliance model.
As for alternative OS, Apple isn't an alternative in the majority of the world. On my home country people earn on average 500 euros, only the upper layer can afford Apple computers.
ChromeOS is hardly practical, and never saw anyone using one in Europe. Just a few shops in Germany trying to get rid of them at any price.
Android might be a solution, but it remains to be seen how the desktop version of Android N really works out in the wild.
As for GNU/Linux systems, I stop considering them as I am yet to see the typical stores that average people go to buy computers invest in a proper packaged whole stack experience.
So this leaves us with Windows, bad or not, that the majority of people already know.