The iPhone 5 was likely to be dropped with iOS 11 anyway, regardless of 64-bit support. It'll be 5 years old at that time. It will still be able to run apps that were compiled with 32 and 64 bit support. It's not necessarily going to be any different to when any previous iPhone stopped getting software updates. I don't see any restriction here on Apple allowing devs to still support 32 bit, they just have to support 64. Someone correct me if I'm wrong.