> On one side having a significantly smaller screen to cater to limits app developers quite a bit, so removing the 320px option would be welcomed there.
I develop iOS apps and love my iPhone SE. It's a bit difficult to develop for that screen size, but it's only a little bit of additional effort and I wouldn't give up my phone for that. Plus, even if iPhone SE went away there are new devices with that form factor (for example, the new iPod Touch) that will be supported for quite some time.
I've noticed this: web apps for mobile and even iOS apps seem to spill over. Is there a tool for helping you to develop for the varying screen sizes? (SE surely is an outlier here, but still, the question stands).
Honestly, the biggest one is just using the device regularly: you'll be able to tell what'll work and what won't a lot easier. It's clear when people design an interface for a device they haven't used (even Apple does this for certain apps). Get a physical iPhone SE (they're quite cheap) and use it as a development device. Or even better, use it as your actual device.
I develop iOS apps and love my iPhone SE. It's a bit difficult to develop for that screen size, but it's only a little bit of additional effort and I wouldn't give up my phone for that. Plus, even if iPhone SE went away there are new devices with that form factor (for example, the new iPod Touch) that will be supported for quite some time.