Which will unravel the App Store.

Because the benefits of side loading on iOS are so massive that everyone will use it i.e. you can use private APIs, bypass Apple's privacy controls, implement device tracking, harvest data e.g. contacts.

It's going to be a huge transfer of power and wealth back to the likes of Meta, Epic etc

Such is life. We should work to mitigate as much of this as possible, but not having sideloading should not be in the realm of possibilities.

(Yes, I'm aware of the workarounds. Those are not reasonable solutions.)

Why can't people just build an OS that can't be abused then.

Just make sure people ask for permission. Before giving them access to something?

We did, it's called GrapheneOS https://GrapheneOS.org

None of those seem like benefits to the iPhone owner; why wouldn't I use the App Store?

I swear to God iOS users just act like Android doesn't exist sometimes

Android has had sideloading forever, yet the Play Store still is the most popular store by miles.

To borrow a popular phrase:

Don’t Android my iPhone.

I left Android for an iPhone for a reason.