I'm concerned many developers will be caught off guard when upgrading to Swift 5 because their app suddenly again another OS versions dependency. While this probably isn't a big deal ("minimum Base SDK < Swift 5 OS requirement" alert), the expectation from past experience is that migrating to Swift 5 has no other side effects than what one might expect from migrating to previous versions.
I would think the better experience for the developer would be, at the very minimum, to opt into using Swift provided by the system. For example, changing a build setting in Xcode labelled "Links against system Swift runtime" (where other state implies the Swift runtime is bundled in the build product as it in Swift 4.2 and earlier).
I would also think the better experience for our users would be to allow the developer to opt in, as well. Improvements are made so quickly and Swift is very young. For example, crashes fixed in the compiled product between Swift 4 and 5. For another, String performance looks to be improving in leaps and abounds. Closures on the stack. Surely we'd want our users to have these with the least amount of friction as possible?
I don't think I'm alone in suggesting I'd rather spend time building features for users with the APIs out of the box made available by people much smarter than I am. Backporting or reverse engineering APIs strikes me as a luxury (or skill) many developers do not have and sure way of introducing security, performance and usability issues. Also, some platforms may not allow the inclusion of custom or updated versions of libraries like
Out of my naivety, I'm not sure I buy the argument that somehow system Swift is needed for Apple to offer Swift APIs, either. For one, there's nothing stopping them from offering a package. For another, I remember hearing (no link, sorry!) they're already statically linking Swift for the apps bundled in iOS 12 (eg: Music.app). The only real advantage to date for ABI stability I've heard is that app sizes will be smaller. I realise I'm arguing this is an acceptable cost to the consequences of the alternative.
Generally, iOS users update their devices. But business don't look at the general population for deciding to drop iOS versions. They look at their user base. Some user bases don't update very much or at all. For example, the IT departments of enterprise clients might not appreciate forking out however much it costs to pay their staff to update all their iPad kiosks to iOS whatever just so the developers can write in latest Swift. Even in terms of the general population, consider Instagram. It couldn't be optimised on iPhone XS because Xcode 10 couldn't build asset catalogs for iOS 9. Do we really want developers not to use Swift 5 so we can get products to the widest audience reasonable?