How does allowing 3rd party app stores negatively affect the elderly? Just like Android the ability to install 3rd party apps and stores would likely be off by default and you'd need to enable it in the settings. And even after enabling it installing something gives you a pretty clear warning that what you're doing could be dangerous.

If that's not enough warning for someone then they probably shouldn't have a smart phone, or if they do it should have strict parental controls on it (blocking 3rd party installs, most sites, and phone calls/texts).