Compare the extremely unappealing offering that is Windows 8 on a junkware-laden laptop with the convenience of a tablet for everyday consumption/casual gaming uses or a much more polished MacBook Something for more serious users, and it hasn't really been a serious contest for several years now. Plenty of people already had a laptop that was good enough for what they needed anyway, and most people looking to buy something new have dramatically more appealing options than the kind of thing Sony or Samsung were selling.
Anecdotal counterevidence:
My mum is almost 50 years old. She has never been computer savvy. All she ever does is read email and browse the internets. Barely browse the internets. She uses her TV cable far more than she does internet.
Last year I bought her a tablet. One of those big fancy Galaxies with a stylus and everything.
Not six months later she started begging me to get her a computer because "This tablet thing is just so useless. I can't type (she got an external keyboard too), I can't tell which app I'm in, what I'm trying to open, and it's just so damn small. I don't like using this stupid thing."
She's been using it as her only computer regardless, but she's been complaining all along about how small it is, that she has to hold it up and a bunch of other things. This year I'm getting her a refurb MacBook Air. We'll see how that goes.
She played solitaire on it, and used Facebook. And I think that was all!
She bought an iPad (finally) and doesn't make a fuss about not changing the font size (she can't, so she doesn't make a fuss anymore), and seems really happy with it. But she isn't trying to create things on it. That could be what's different with your mum.
I should check how much they still use their PCs. My dad used to program a lot, and my mom moderated a mailinglist and was active in a bunch of communities (for some context: I'm 40, my parents are 68). Tablet usage is definitely more passive and consumptive.
ASK her before if she'd rather have an old 15" MacBook or an Air. If she's complaining about the tablet being to small, she'd probably rather have a larger screen than 13".
If someone had the answer to that question then Microsoft, Sony, Samsung, etc. would be much, much happier. The entire PC industry has been shrinking for the last few years, sometimes with double digit declines. Nobody wants a heavy, hot, loud, and crappy laptop when a tablet like an iPad can do everything that matters to most people (email and web) and then some.