There are several Linux OS's for phones, but having to install them yourself on an old phone just isn't going to bring about wider spread adoption. It's too scary for most people.
Having a phone for sale with a Linux OS is a great first step, like a proof of concept.
I have a Pur.sm Librem 5 Linux Phone on pre-order. They keep moving the release date back but currently it's set for 3rd quarter of this year. It's pricey but I want a little more horsepower than the PinePhone.
With desktop use falling away, Linux needs to get into mobile to continue forward, and we need Linux to breakout of the Android and iOS monopolies on mobile operating systems.
