Do you think suddenly iphones will come out of the box with competing app stores?

What made this strange thought pop into your head?

I think they'll visit sites and instead of being recommended to install an app, they'll be recommended to install an app with a redirection to a different app store.

Apple takes a huge cut of the money spent on the app store, and everyone will have a huge incentive to move users to different stores if they can. This isn't difficult to understand if you thought about it for more than a minute.

That would be apple's problem then for making their store so unattractive to developers. What you're saying here is that if developers had a choice they'd tell apple go fuck themselves and you're probably right. "We'll engage in anti-competitive practices so our customers can't escape our shitty offering" is not the flex you think it is.

Ok, and then it won't work unless they explicitly authorize a new app store to be installed.

Android supports it, but it's not just 1 click from a browser to get a new app store.