Well, you can still have breakpoints, and switch between different structures for e.g. mobile vs desktop, or landscape vs portrait.

But I don't think you can cover all requirements with a set of fixed layouts, they do need to be somewhat flexible. Even if all screens were the same size, you'd want to be able to boost the font size for accessibility, and that essentially means scaling the whole UI.

And the slightly different sizes of iPhones are not really that problematic with something like autolayout.

I think something like flexbox is a better fit. There are tradeoffs in each approach, but I usually find a local, modular, bottom-up approach easier to work with than global constraint solving, even though it seems in principle like it should be nice to be able to say "keep this button here in relation to this other button". As you add more constraints like that your layout slowly turns to mush and doesn't actually resize nicely. (Edit to add: I'm probably conflating a few different generations of iOS toolkits here, I realise autolayout is somewhat separate from constraint-based layouts.)

I don't think it's a coincidence that most other UI toolkits have added something like flexbox (including iOS) -- it's not perfect but it fits how people generally think about UIs. Was HTML/CSS the first major UI framework to use flexboxes? That's how I remember it, but maybe it was copied from somewhere else.