As long as you are available to be on-call to fix literally all of their support issues:-). That's an old joke, back from the time people used to have "printers", and so like on a Windows and Mac machine adding a new peripheral is trivially easy whereas to install a new printer/scanner/whatever on a Linux machine, even if it is as simple "apt get whatever" can be quite intimidating to elderly people (tbf the OP does say "parent" and not "grandparent", but I was thinking both).
Anyway, that's a good point that I have no fully thought through yet:-).
True... though Windows itself keeps changing things between major releases, so someone may feel that they need to learn a new interface either way, which lowers the barrier to such a switch-over. For myself I did not think it was that hard, but then again I came from Linux to Mac OSX so I am definitely not the average use-case:-P.
Anyway, good point.