By "one runtime only on the target device", do you mean that ABI is tied to OS version, and any time the ABI changes, you rev it with the OS? And if so, and the ABI changes, does that mean that customers need to go and redownload every single app they own (after the developer has gotten a chance to recompile it for the new ABI and upload to the App Store)?
Alternatively: if the OS is expected to support multiple ABI versions, does that mean that it ships with 10x versions of every single system framework that uses Swift, so customers can download apps with different ABIs?