I'm also considering the transition in the Steam marketplace as a recent example. Their opening from curation started with Greenlight, a fast track program with some but minimal curation. There were a few turds but by and large the games that came through were of some general quality. Enter phase 2, Early Access. The minimal barriers were removed except "will it run", adult content allowed, and a smaller hosting fee used. And in came the parade of low effort hot garbage. Recommendations in their platform are hard to come by now. Random impulse buy while scrolling rarely happens for me now as discovery of actual good games for me is much lower and I feel like the platform as a whole has suffered for it. I could see a similar trajectory for the app store albeit with my value of the services being placed in different categories.
I would think anything that required Apple to open up the OS would apply equally as well to carriers. Who cares if the carriers put crap on your phone if you can easily wipe what they provide with a clean copy? It's slightly more complicated than Windows in that the carrier is also providing drivers, but not entirely without precedent (much of Dell system innards are their own design, and they re-brand or develop their own drivers for chipsets to better suit their systems). That would be another differentiator to open the market though. IF X provider is hostile to replacing their OS and makes the experience suck, the market can deal with that. At least it's a small chunk of the stack, and not everything from the base hardware to web services all locking you in to one choice.
As for Early Access and opening up Steam, i've actually not found it to be a problem. Some of my favorite games and experiences started out as (and in some cases still are) Early Access. And even the ones that started strong and went to shit, I count 6-12 months of a fun game as well worth the $20-$30 an Early Access game generally costs.
I've also found the Steam ratings, and the reviews people put in that generate it to be extremely useful and accurate. You just have to zero in on the reviews that relate it to existing experiences that you're familiar with so you get a good idea of what it is like. Worst case you find some streamer on Twitch or YouTube to watch for a bit to get a feel for it.
If Facebook had a worthy competitor for example, surely you'll agree Facebook would be a considerably different product now. You might say that doesn't help my case, because it would actually be a better product, but it's not an inevitable outcome in every comparable scenario.
No, I don't think I would. The vast majority of people are not going to leave facebook as long as they can't take their social contacts with them, and Facebook knows this. They'll let you have your pictures and videos, but the graph? No way, and they know most people won't leave entirely because recreating all those links is a large undertaking for most people. Even young people that originally shun Facebook as their parent's social network eventually join because the network effect is too large to ignore.
The fact that Facebook very quickly buys anything that looks like it might have a draw that could possibly affect this in any way doesn't help it. Neither does that they blatantly lied to congress about things they would/wouldn't do with some of these acquisitions when asked about it.
There's no reason to expect it will be exactly like a PC, which is coming from a completely open past to a future which allows more locking down. The iPhone is locked down now, it's a bit ridiculous to assume they would immediately go straight to allowing anything and everything to be installed without any hoops jumped through.
Even Android requires you to allow unsafe sources to isntall third party packages. Why would anyone expect the iPhone to go farther than this when they're fighting tooth and nail to not even do this much?