I think there's two types of sideloading that usually inspires fear:

1. The fear of the other big tech giants creating their own app store, forcing users to migrate there to use Facebook/GMail/Amazon/Outlook etc., and then siccing invasive tracking on them for the purposes of serving ads and selling user data.

I think this threat is easily more conceived than realized. While the motivation exists to have easier access to user data, I don't think users will necessarily bite. Having to join a new third party app store just to be able to use Instagram or WhatsApp would alienate, and anger, most users. It's yet another account one would have to manage and keep track of. It breaks the seamless nature of iOS and mobile experiences in general. Everyone would know why they're really doing it.

And these companies are big and old. To create a competing app store takes a lot of effort. Not just on a technical level but on a product and business perspective. To convince users that this added friction is worth experiencing. Based on the anemic state of the Amazon Appstore or the Samsung Galaxy Store app stores on Android, tech giants only end up investing in alternate app stores if it's already tailored for the devices they make, which is a moot point with iOS. And those stores aren't even dynamic, nor contain app exclusivity as far as I know.

Not to mention, running a third party app store means that Meta will have to start wooing other third party developers so it doesn't just end up being the Facebook/Instagram/WhatsApp show. That means they have to start creating their own pocket ecosystems and manage their own platform communities as well. It's a lot of hassle and effort for something that won't necessarily pay off. Just ask Microsoft how their Windows Phone store fared. Or heck, just take a look at the current state of Facebook apps.

Maybe ten years ago when mobile apps were still a fairly new category of products, when there was a lot more room to grow and these companies were less entrenched and nimbler, there was a chance that rival stores could pay off. But nowadays? Sure from a technical perspective they're all capable of building their own cloud gaming service or Clubhouse clone or Snapchat stories knock-off, but how many of those actually stick?

I think the only companies that will want to heavily invest in their own third party iOS app stores will be video game publishers.

https://news.ycombinator.com/item?id=30204012

2. Installing random app binaries. Dubious files, email attachments, malware, random scripts, all common vectors for viruses and computer problems.

This fear on mobile is overblown because Apple still controls the operating system, and can carefully set the flow to prevent easy access to sideloading. They can enforce all sorts of OS and app binary-level restrictions even without needing to go through the App Store.

https://news.ycombinator.com/item?id=30199125

Though now that I've laid out the two topics, I suppose one potential worst case scenario is the tech giants act greedily and stupidly, force essential apps to be downloadable only on their third party app store, so users resort to sideloading those apps from shady sources.

But on the other hand, maybe people will use resort to mobile web instead. Or just quit using those apps altogether. We're already see Facebook adoption drop precipitously.