The swipe you do to go back is very short as it's from the side of the screen, unless you choose to drag it, but for me it's a quick move from any side, and if you use the built-in SwiftKey you can adjust the keyboard position.The 3 button nav is faster to me too.
If there's an app where you have to go back a number of times, it is no where near as fast to swipe as it is to just tap the button 3 or more times.
A tap is faster than a swipe.
I also think that the on screen buttons make it easier to type as the keyboard is higher due to the buttons always being there. When the keyboard starts at the very bottom of the screen, it becomes cumbersome to use (for me).
So I'm glad they're still an option.
But yeah, different strokes and all that

Best thing about swiping to go back is you can do it from any side and from any position. Could never go back to the 3 buttons, feels ancient and a waste of screen space.
I'm guessing you don't use SwiftKey because you can position the keyboard to come on higher up the screen if you want.
Exactly this.
I simply cannot go back to the 3 button nav keys
