OK, I haven't actually taken this approach with either my parents or my wife's mom :-) My parents bought a Mac back in ~1987, and continued to upgrade Macs occasionally. My mom's Mac is currently secured by the fact that it's a Mac, and she only uses dial-up internet because that's good enough for email and she doesn't see well enough to use the web unless she really needs to. (And my siblings all grumble about not having decent internet access when we visit, but she really doesn't want to bother upgrading, even though she did finally get cable TV when the digital transition broke her ability to receive the PBS station she likes via broadcast.)
We got my wife's mom a generic Windows PC when she retired, with AOL. It let her chat with her friends, keep up on the celebrity news, and generally stay connected to the world when she was getting less mobile. "Computer security" for her system meant occasionally formatting the disk, reinstalling Windows from scratch, reinstalling a new free AOL coaster, and having her log in to AOL, because she kept everything she cared about in the cloud rather than on her PC.