I don't understand why people say this.
Even if Apple allows sideloading and such, 99% of people will still install everything from the app store. It will be up to each person to decide whether they want to use an alternate app source.
If you want a curated, safer environment, stick with the app store. Believe me, companies like spotify will still be there, just as they are on Google Play even though Android can sideload.
There could be a switch in the OS to enable sideloading, with a warning when you flip it. Most "regular people" will never touch it. Not only that, but apps from third party sources will still have all the protections from Apple's sandbox environment and system security. Those don't change just because an app was sideloaded.