I too like the walled garden. I don't really see that as an argument against sideloading however. Presumably the first-party app store would continue to operate with the same restrictions and I'd happily avoid opting-in to sideloading.
The people who want it are happy and I'm no worse off.
The App Store is a forcing function for app developers, because it's the only way to reach IOS customers. Without that forcing function, some app developers will circumvent, and if a critical mass does it, then you no longer have the option of opting out.
Apple can presumably make the warnings scary enough to make app developers reluctant to leave, but alternative less restrictive app stores and big-name developers will lobby to have those warnings removed for being deceptive.
If some game announced you could only get it through an alternative app store, I wouldn’t play that game.
Banking is critical functionality on the other hand. Under those circumstances, I’d change banks. The likelihood of any major bank attempting that seems awfully low and I’m sure it’d attract the widespread condemnation it deserves.
Android has sideloading. I don’t doubt some apps try to push users to an alternative marketplace but important everyday apps are where you’d expect them to be.
Apple should not have that kind of power over owners of the devices Apple manufactures.