You're envisaging something like a medical or banking app that is intentionally not on the App Store, but instead requires you to use some other installer?
I guess that's possible, but seems a bit unlikely -- it's just a pretty big barrier to entry for your users.
Android technically allows this already, but how many major apps are not on the Play Store? (Apart from Samsung apps, which is slightly different case as their store is preloaded when you buy the phone. But there won't be a Samsung iOS phone any time soon.)
I think it's likely enough I expect to see it not just happening but also going wrong within 12 months of this change taking effect, assuming 3rd party app stores are not also regulated to actively prevent that (which they may well be). There is also this anecdote of basically this problem happening on desktops: https://news.ycombinator.com/item?id=32166035
I guess that's possible, but seems a bit unlikely -- it's just a pretty big barrier to entry for your users.
Android technically allows this already, but how many major apps are not on the Play Store? (Apart from Samsung apps, which is slightly different case as their store is preloaded when you buy the phone. But there won't be a Samsung iOS phone any time soon.)