How can AArch64 be as efficient when it implements all of the old 32-bit extensions?
Some don't, but a phone does.
Some don't, but a phone does.
Google has purged 32-bit apps from the official Android app store, but as I understand it the Chinese OEMs that ship un-Googled AOSP ROMs with their own app stores haven't been as aggressive about moving to 64-bit.