After just watching the excellent WWDC session by @Ben_Cohen (Swift concurrency: Update a sample app - 10194) its been made very obvious that by not having the ability to back-port Swift concurrency by even a single major iOS version is a multiplying problem across the industry.
Let's take our team of 15 iOS devs as an example. We currently support iOS 13 as our minimum deployment target and have over 220k swift LOC in the app. The minimum deployment target is dictated by our users upgrade behaviour, so as it stands we won't be able to drop iOS 14 until iOS 17 is out. In that time a lot more code will be produced that can't adopt Swift concurrency and we are possibly looking at double or more lines being added to our codebase over that time.
The problem is that the more code we add over that time the more time it's going to take us to adopt async/await and the harder sell it's going to be to the business to have time to make those changes. It's been made very obvious in Ben's talk that this isn't going to be a trivial conversion and will take a long time to adopt.
All the time we spend writing code for the next two+ years we know at some point we are going to have to do some major refactoring. So then comes the question; should we just continue to use GCD and pretend swift concurrency doesn't exist for the lifetime of the app? From a business point of view the time we spend actually fixing concurrency issues right now pales in comparison to refactoring the entire codebase to adopt Swift Concurrency in a few years time, and that balance won't actually shift for a long time. This is something that will be echoed across the industry, hence the multiplier I mentioned before.
I think the Swift concurrency features are great and I've personally played with a few of the early snapshots, so it came as quite a surprise that it wasn't being supported on older iOS versions. I have a feeling that we have missed the boat already to get this changed for the release of iOS 15, but it's worth saying that if this could be supported for even 1 previous major iOS version then it is going to save many thousands of hours of developer time over the coming years.