Sorry, but I still don’t buy it.
Okay sure, you can instruct the client with some form of metadata or markup that the login form now requires an age field. Does the client know already how to render that? Is it a picker or a text field? Well, no problem, we just stuff as many primitive UI elements as we can think of into the client (like a generic number input we can use for our age), so we can dynamically compose its UI. Congratulations, you just reinvented HTML.
Which raises the question: why didn’t you save yourself a lot of time and you just use HTML in the first place? iOS does allow you to do that, after all.
Here’s a guess: because HTML has horrendous performance, and it does so *because* it tries to accomplish this very thing. That’s the reason Apps moved away from that approach and went native in the first place. That’s why Webapp frameworks like React or Angular have started to bastardize HTML in a desperate attempt to get away from that nonsense and emulate native Apps as much as possible.
Webapps show that there is also a much more reasonable and efficient way to be just as flexible: just allow clients to update their code instantly. The fact that Apple puts a one week barrier in front of you is not a technical or conceptual limitation, and can in fact be bypassed quite reasonably with React Native.
Could this be done better? Sure, but REST isn’t the answer to that. It has already failed with what it set out to do a long time ago, and trying to bring back the glory days of static webpages (which were as RESTful as you can get) is just a pointless round trip to the past.