I see no reason Apple would prevent people from continuing to upload dual 32 and 64 bit apps for now, as they currently do.

Depends on individual developers and how easily Xcode 9 will let devs target older devices.

For example, the iPad 2 can't upgrade to iOS 10, but plenty of apps still support iOS 9.

The iPhone 5 is four and half years old. He'll be fine. • He probably won't be able to update to iOS 11. • He'll still be able to run iOS 10. • He will still be able to download new apps. • He will still be able to download his old apps, even 32 bit ones, because he owns them.

Yep. Nor will he be able to get the latest version of iOS. So just like any other upgrade that removed older devices from the supported list.

Based on what evidence? Apple still accepts 32-bit slices if you want to submit them. 64-bit slices are required.