Sorry that happened to you. I have worked with a lot of elderly people in the past and it is always a shame when that happens to them. You are right that you can't really "fix" them. Even if you lock down iMessage and prevent sideloading, scammers will still send them to phishing pages in their browsers, or get them to read out a gift card over the phone. These methods are actually way, way more common than getting them to install a malicious sideloaded app. Ultimately I think Apple's anticompetitive tactics had no bearing on your grandmother being scammed.