Hello, well in the developer preview we can see that there is still an option to use the normal on screen navigation that was present in the previous version of android. However, I see two ways here. Either Google won't use the gesture navigation feature from devices with physical menu button or they can provide the option to use the gesture navigation and disable the physical menu buttons or maybe just give an option to use both gesture and physical buttons at once. I have heard other people suggesting that Google might be able to use gestures in the physical button itself like you could swipe the fingerprint sensor just like the on screen gestures. That could also happen but I see less probability of that. Thank you.