Developers won't optimize UX of their apps for phones, unless they'll see valuable target audience there. And this isn't going to happen without selection of new devices of market (their current best one is so inferior to iPhone 7, that the only excuse to buy it is the price). So what we'll get is a lot of ugly apps that barely work on small screens.
Don't forget though that these apps need to support split screen view and other things.
When you get down to it, at the more extreme ratios, split screen is just like a really, really tall phone, and developers are used to dealing with tall UIs on short screens (scrolling).
