it's entirely possible to be an adult who has had to sew clothes for their family for years and still be unable to turn out anything except amateurish assembled rectangles.

You're right. I stand corrected. I do assume too much in saying that 'anyone can do it' when that's not exactly true. But it does mean that your grandma probably did not want to sew once she didn't have to, ie, once it's possible to buy them or have someone else sew them for you. That's probably where your point about interest in the activity comes in. I was assuming that anyone who actually wants to sew would be able to make something nice given sufficient investment in time, etc.