I was the same. The moment I saw the three icons gone I rolled my eyes. I thought it was pretty user friendly as is. That said once demoed I am interested to try it. I would like to have an option to toggle back though if desired because...
When I see UI changes like this my initial concern is not for myself but for my 75 year old mother. It has taken her years to be able to move through Android with some efficiency and the basic consistency has been a good thing. A fundamental UI change like this isn't as easy for users like that to adapt to. I am already dreading the first time she has a Google P based phone. I hope to be proved wrong.
How are UIs on third-party apps affected? If you turn on gestures, does it mean that on a third-party app, one has to use gesture now to do what could be done before using a back button?
If you turn on the gesture mode, it's still largely the same UI, it's just that the "App switcher" button is gone. The Back button is still there, in the same place as before (although it does disappear on the Home screen now, which is neat).
Tapping Home and tapping Back still do exactly the same thing. Long-pressing Home still opens Google Assistant.
Opening the app switcher UI is now done by dragging Home up (instead of tapping App Switcher).
Quickly switching to the last used app is now done by dragging the Home button right (instead of double-tapping App Switcher).
When I see UI changes like this my initial concern is not for myself but for my 75 year old mother. It has taken her years to be able to move through Android with some efficiency and the basic consistency has been a good thing. A fundamental UI change like this isn't as easy for users like that to adapt to. I am already dreading the first time she has a Google P based phone. I hope to be proved wrong.