I'm still not following your math. On a clean phone none of those 32bit libraries will be resident in memory, since all of their own apps are already updated.

I'm still not following your math. On a clean phone none of those 32bit libraries will be resident in memory, since all of their own apps are already updated. If a user starts an older 3rd-party app then that process will incur some overhead, until it exits. Since Apple's frameworks have already been through this transition on the desktop, the average 3rd party app will likely make this transition easily with a build target setting. I'm sure older games, like the original Infinity Blade, may never make this transition. I doubt any users will notice, though.