To all the non-devs out there: this article is misleading. Devs can still target older iOS versions, thereby ensuring compatibility, they just can’t use the old SDK to do so. There’s really no downside for anyone here.
Well, there is kind of a downside for devs. It means if they don't care about the X layouts, they still have to write conditional statements anywhere anything is handled differently for iOS 11, like the safe zones for the X. Conditionals/exceptions like that just bloat and complicate code.
Also, forcing them to compile with Xcode 9 means some of their Swift 2 code and libraries may break. It also forces them to be running at least macOS 10.12, which they may not want to do, and for which there are many legitimate reasons not to.