Not to pick on you, but why when the question of Linux as a Windows substitute comes up, do people always have to offer anecdotes about their mother. I'm no SJW or radical feminist, but honestly, let's move on from that.
My comment is about "the average worker", not friends and family / personal use. For which, in my highly uber technical group of friends and family, (where mothers are C++ programmers), maybe 1 out of 20 (generously) find a Linux desktop usable.
Only in this forum, where people recognize the horrid low contrast, gratuitous whitespace, excessive bloat of the latest javascript "framework", know the difference between modeless and modal dialogs, understand why the gmail UI keeps moving in the wrong direction -- only in this forum, where UX is understood as a real "science" that needs attention, will people actually also claim that the Linux desktop is usable. It boggles the mind.
Sorry to go off on a rant, forgive me.
It's just a matter of familiarity. She just want to check her mail et watch videos, after all.