It's worth mentioning none of these concerns affected Android / Google Play, even though it's fairly easy to sideload and even install custom app stores there.
The only phones that come with alternative stores are from Chinese manufacturers, which isn't going to be an issue for Apple as the operating system isn't open source.
The OPs main point was that the existence of sideloading or third-party app stores hasn't led to major security issues. And apps still have to comply with the sandboxing.
