How? There are many viable alternatives to everything Apple makes. There are plenty of non-Apple laptops, desktop computers, tablets (ok, this is more limited at least in terms of equivalent capability), phones, even the watch.
And Apple, unlike MS at the time, has not been conducting the same abusive policies that put competitors out of business, which is a large part of what raised the cost of entry.
>Third, and largely as a result of that barrier, Microsoft's customers lack a commercially viable alternative to Windows.
Do Apple customers have commercially viable alternatives? Apple's current customers and, not only alternatives, viable alternatives?
I personally know many people who lament, "All my stuff is Apple, I can't leave!" Of course they could leave if forced to but it is not viable. Perhaps their other devices would stop working. Those people aren't choosing Apple products because they think they're better, they are choosing Apple products because they are a victim of Apple lock-in.
I remember when Microsoft had its dominant position with Windows[0] in the late 1990s; sentiments along the lines of "this sucks, but we're stuck with it" were common. I do not hear that sentiment from Apple users often today. Even when they have complaints, they typically like the hardware and/or software better than alternatives to which they have been exposed.
In case that seems like something a fanboy would write, I should clarify that I am not an Apple user, nor do I have any desire to buy Apple products (though I do hope somebody else manages to build a fast, cool-running ARM laptop soon).
[0] Windows still has a very high market share for desktop operating systems, but there's much less lock-in for most users.
And Apple, unlike MS at the time, has not been conducting the same abusive policies that put competitors out of business, which is a large part of what raised the cost of entry.