Re: "concern that punters would [..] lock themselves out of their backups"
So you it not the default. Everyone using iTunes for backup and setting a password has the same potential for being locked out. I'd gladly accept this as the price for being able to use the more convenient iCloud for device backup, but until it is encrypted as well as iTunes backups I'll keep using those.
I mean, if being locked out of your backup is a concern, why not have a concern for locking yourself out of your phone. If you forget your password/passcode, you are just as locked out. Whether companies like Cellebrite can possibly unlock it for you using exploits Apple hasn't patched yet is irrelevant to the average user who isn't going to hire some company thousands of dollars to unlock his phone for him.