I thought it might be worth addressing the "obviously" at the beginning of your question.
Developers do not, in general, acquire every Apple device that is going. It is perfectly possible to develop software for the whole range of iPhones from 6 to XS purely by having one test device and simulating the rest.
The main exceptions to this are if you are using a hardware feature that an old device doesn't have. For instance, if you seriously depended on Force Touch then you would have to have a 6s at the very least. But simple things such as the changes of screen size and layout that accompany new devices can usually be handled perfectly well in the Simulator.
The only times I have deliberately delayed updates until I had the device in question were the very first iPad and the very first Apple Watch. This is because the user experience was different enough that one couldn't imagine it without actually getting one's fingers onto the device itself.
As for your question about phones, I don't want to get into that because I am not an expert. But if I just buy a phone then it doesn't become my main phone unless I specifically set it up as a replacement for my old phone in iTunes and specifically move my old SIM card to my new device. I don't have to do either of these things.