But the phone worked "as well as it used to" on iOS10 with support for 32bit apps. So why would it not work as well as that? I think what will happen is the user don't understand why their precious legacy app doesn't launch at all.
Also, they could just continue to pop up an alert saying "this app may slow down your phone" like they already did in iOS10.
Also, I doubt there is much of a performance hit anyways. For example, the current facebook app is 64bit and it is a slouch to start.
The simple solution is for developers to recompile the apps. The developers of any apps that are truly still needed by customers, can easily convince those customers to pay for the update.