In my case, I don't get rid of my old phones, and I have one with a data SIM card that I keep loaded with minimal apps, only the bare necessities. It usually lives in the car on long road trips (literally--it stays plugged in, in the center console, for a few weeks, to provide nav to the car's head unit), and it would be nothing to put it in either of the holders on the bike to provide navigation. I wouldn't use my good/main phone for this purpose, but it's not a big loss if the older phone gets damaged.

It's too bad someone doesn't have a remote screen for a smartphone, similar to how Android Auto works in a car. You tuck your phone safely away, start up an app called Android Cycle, and it projects video and sound to a rugged remote screen with your TbT directions. It could even be a "digital paper" LCD screen so that it is readable even in bright light.