Previous versions of Swift allowed static linking of the standard library and runtime on macOS, but that's not going to work anymore because parts of the OS also link against the standard library and runtime, and the two wouldn't be able to interoperate. (We had to jump through hoops not to break Swift 4-and-below apps, but we know that those aren't trying to talk to any of the Swift parts of the system, because there were no Swift parts of the system when those were deployed.)
We're in an inconvenient spot now, but in the long run this won't be a problem.
Yes, which is why you also need to have a Runpath Search Path for /usr/lib/swift/.