Sideloading is still possible and is free, but you have to know what you're doing. It's a tremendous barrier to entry (your keys even expire every few days so you have to keep re-sideloading) thus people don't bother with it.
Alternative appstore is basically same as sideloading, cause the latter is necessary for the former to function.
Btw important overlooked issue is not only sideloading per se, but unrestricted access to push notifications / subscribing to alternative push notification services. With restrictions on running in background iOS apps are next to useless without notifications - you won't get VoIP, email, chat apps working without capability to wake up on push.
I'm very happy in my locked platform, I'm happy that my family are on a locked platform.
If they (or I) want to be on a more open platform then that exists, just not for apple devcies; but for now I trust random developers less than I trust Apple.
If it becomes normalised to bypass the app store, then it's going to be very difficult to control the quality of software generally; and on a macro-scale will entice even large developers (like banks) to go the 'alternative app store' route, which will force everyone to be using applications which are not vetted in a reasonable way.
But why are you wording this as if AppStore will somehow vanish? I don't think it will be going anywhere even if some apps will start working without Apple's permission. Chances are, it'll get better if it has some real competition.
It's like buying an android phone then being sour that photoshop doesn't work, I think.
As for why I'm wording it as if the AppStore will dissapear, it's because of all the reasons I laid out.
Right now it's a gate or a fence; the great filter which all must pass through; but if there exists a cheaper/easier alternative that gives the same results, why not use that? It will become incredibly normal to run the alternative app store- to the point where a lot of software will be released only to run there (which is a stated aim) and if everyone has the alternative store, why bother with Apples?
Yes, for a developer or advanced user, walled gardens look bad. But we as a community need to accept that we are a small minority of users of Apple devices. I personally recommend iPhones and iPads to entry level users in my family because I know they can't mess up the device completely without knowing ... so, to be honest, because of the walled garden and the limited choices. And I accept that if I want more freedom, I have to get something else for me.
Therefore, the question isn't why someone doesn't want to give you the choice and freedom, but why you (and sometimes I, and a lot of people in our community) want to force a change to the majority of iPhone users that are perfectly happy with the current status quo. A status quo that was in place 5 to 10 years ago and was definitely in place when they bought their devices.
Or, maybe, you don't like the idea that you'll have to sideload the app you need because it isn't available in the AppStore? Hmm. Well, you'll feel a biiiiit like other people who don't have a choice under Apple's regime, with only difference that they can't run the app at all, and you can run it in a way that you don't like.