Windows is still around, I think. And you used to be able to install anything on it, from any source, with zero control by MS. (On newer versions that's possibly less true.)
But if you want to steal grandmas' money, it's much easier to setup a romance scam operation than to try to get them to install some app.
You can search for innumerable articles and anecdotes of people moving their family and elders to iOS on mobile and Linux on desktop for exactly these reasons
