It'd probably end up working like with old CDMA phones where you had to switch the active IMEI of the device on the carrier's website.
Those same CDMA carriers (like Verizon and Sprint) that have a
policy of not allowing you to bring a device that isn't theirs onto the network? The ability of those carriers that you're talking about is holding back competition in the USA.
If the carrier's website or phone line is involved, they'll have the ability to say that you can't switch for some reason. Whether that's because your new device isn't "compatible" with the carrier, you've switched too many times in the last 30 days, your plan doesn't allow it, you have to wait until you're x months into the contract etc.
With a SIM that simply isn't possible. My current carrier has no power over when I take my SIM out of my phone. Likewise, a new carrier has no real control over what type of phone I put my new SIM into.
I'm quite worried that some carriers would require you to always have the SIM associated with a device. So if you want to temporarily stop using your main contract while you go abroad, you'll need to swap the service to another phone temporarily.
This system would also be totally useless if you don't have a signal. It'd be a total non-starter if you were going to a different country and hoping to buy a local SIM in the airport.
You need to change SIM if your phone is out of juice, or you want to lend your account to someone else.
Also, if I change my number or I have a long-term contract, it's convenient to keep the SIM in another phone as a backup or pass the SIM on to someone else. If I have bought a special-purpose SIM (e.g. when travelling abroad to get a good deal on data), I want to be able to put this into my phone. I may also want to borrow a friend's SIM (e.g. when travelling abroad). All of these will probably be blocked by an e-SIM. These are common things to do in Europe.
Exactly. There are so many scenarios that the physical SIM enables in a competitive telecoms market, but Apple doesn't see that in the USA.