I agree with you on Touch ID under the screen being cool. It works so flawlessly, and I have confidence they'd pull it off well. They won't do it if they don't feel it's ready though. That's how Apple works, and I'm fine with that. I would rather have a new feature working as well as Touch ID does now instead of something half-baked like other manufacturers. It's not always about who's first to the market. Sometimes it's about who's best in the market.
I think if they went the route of having Face ID being the only means of authentication, it's probably safe to say they feel there is more potential for Face ID. I love Touch ID, but I'm actually very excited at the prospect of Face ID if it were pulled off correctly. If it turned out to be as quick and accurate as Touch ID, I'm all for it. Imagine if Face ID authenticated you before using Apple Pay and provided a small window of time for you to hold your phone to the terminal. This is similar to how Samsung Pay works. You can authenticate with your finger then hold the phone near the terminal without keeping your finger on the fingerprint scanner as you have to with Touch ID. If Face ID worked in this manner, it wouldn't be as awkward as some people are worried about with respect to having to hold your face in front of the phone while holding it near the payment terminal at the same time. I feel a Face ID implementation similar to this would be easier than Touch ID currently is. Every now and then if I hit up a McDonald's drive-thru, keeping your finger on Touch ID while the cashier is holding the terminal out of the window is certainly an awkward angle to hold your phone at at times. I often feel I'll drop it if I don't have the right grip on it. Face ID in a manner how I described would eliminate this concern.
It will only be a matter of time before Android phones remove their auxiliary ports as well. The Google Pixel 2 is already rumored to be potentially removing the auxiliary port after touting it as a feature they kept while taking a jab at Apple for removing it. We are moving toward a wireless future. It may be a while before Bluetooth headphones catch up to the quality of most wired sets of high quality headphones, but who ever thought cell phone cameras would be taking pictures with quality to compete with point and shoot cameras?