How easy will they let it be to send your eSim from phone to phone? Say my phone decides to stop working one day (this has actually happened to me in the past with an iPhone 6, while on a trip to Myanmar of all places), but I still need use of my phone number for work or personal reasons... do I now have to go and buy a new iPhone or repair mine? Rather than just grab a $15 throw away phone to tide me over for a few days? Or perhaps swap SIM's over briefly with your partner's iPhone? If it takes longer than physically swapping a SIM card, it's a fail.
How easy? That depends. Apple is doing the best it can to make the process easy and convenient. iOS 16 introduced a new eSIM transfer features that lets you kind of “AirDop” an eSIM but the carrier needs to support it first.
GSMA Standard sadly did not include such a feature so Apple had to come out with way to do it and that means carriers got to work with Apple to make this feature happen meaning it’s an iOS exclusive at the moment.
If your carrier does not support the iOS 16 eSIM transfer process then you will need to check with your carrier how to move eSIMs to a different device.
For me, my carrier offers a QR code in their app if it detects that the eSIM was removed. That means I can delete my eSIM right now and my carrier app will instantly show a QR code to re-download the eSIM whether on the same device or another.
If my current device is dead and I cannot remove the eSIM then I can call/email/WhatsApp my carrier and they will drop the eSIM from their end and email me and text me (on another number) a new QR code after verifying my details over the phone.
The process is heavily dependent on what carrier you’re on though. Some can make it easy and support the iOS 16 feature. Some make it tolerable like mine and some may make it a nightmare and ask you to visit a store to get a QR code printed on a piece of plastic.
Now comes the device support question. I see your point. With a physical SIM you can just buy any cheap phone and use it temporarily. Even if it uses a different sized SIM (like micro SIM, or mini) you can just get an adapter for cheap or perfectly align it yourself and get on with your day.
That is sadly not possible with eSIM. With eSIM they cheapest Apple device will be the XR/XS. If we look at the Android side then it is one whole other mess because not many do eSIM yet and even those who do, some of them can’t even make it work yet like Samsung.