You're not wrong of course - this would come with heavy UX issues though. E.g. it's hard to type 10 words into a password field without messing up. And _really_ hard on a mobile device.

1P would allow you to have a password like "1234" and still have the same key entropy as that 10 word password alone.

A mobile app could easily facilitate that by storing a "passphrase prefix" (or suffix, etc) using biometrics/device key and requiring users to enter the remainder if you don't trust the device to store the entire thing.