A good place to start would be a quick poll on what percent of people actually password protect their phone and would benefit from a 'faster' finger scan.
Also, bleeding market share to larger screen devices would be a good indicator, too.
Whatever they add, I just hope it's not visible in the digitizer.
Every single person I know has password. Except me, I hate to type codes just to unlock my phone.
And if security was painless, more people would do it.
Most people wouldn't buy insurance if it wasn't required in most activities.
Apple has made a 'no' decision to focus on portability and discrete 4, 8, 10, 11, 13, 15 inch sizing for mobile, assuming that those are the critical few for sweet spots. Yes, you'll get closer to 100% coverage if you fill in all the gaps, but that's SKU drift that Apple doesn't want to excurse into.
Now... a 'no password' security system (first fingerprint, then true voice/facial recognition (linking voice/face and movement) is painless security (except in below zero climates like MN, where a bellaclava and gloves may make it problematic ;-). And that is critical in moving to a new economic system that is both cashless, and signatureless:
Something you have (a device)
Something you are (your face, voice, fingerprint)
Something you know (the final, vs the first level of security... the secret).
For 50 years, passwords have been the first level vs the last (typically, security is 'where you are', 'who you are', 'what you know [secret phrase]'. We can get back to that, starting with fingerprint (requiring a warm, pulsing finger, not a picture of a fingerprint, but a 3D and multi-layer (surface, subsurface, and capillary)). Couple that with a few years of being able to get to voice prompting (Please say your name and passphrase), and then a 3d facial recognition (again, defeating paper pics), and we are on our way to a level of 'consumer security' that will greatly reduce the losses that 'ASCII passwords' now expose us to as an 'only form' of online security.