What would take for GNU/Linux to at least be viable for mobile?

First there's the hardware. There's basically two options, either a Linux system can be installed on an existing phone - this is difficult, because bootloaders are often locked down, and drivers aren't open source. So there's an extra effort to make such a system compatible with existing phones, usually ends up being a specific subset of Android phones which play a bit nicer than others, or just are more readily available. Laptops and PCs are a lot more generic, being already compatible with a standard Windows release, while phone manufacturers will generally build their own custom version of Android with necessary tweaks and drivers built in, so it's not so easy to replace with open source software. The alternative is building a the hardware alongside an OS, as we've seen with the likes of Pinephone and Librem. But this is a lot of extra technical complexity, and very expensive to do at the scale that they'll be producing for a niche product. So you either end up with something like the Librem which is as expensive as most of the highest end smartphones, or the Pinephone which is cheap but has quite low end hardware. And with the time it takes to build such a device, usually the hardware is pretty outdated by the time it's ready for prime time. Then there's software. For one, most existing Linux software isn't well optimised to run on a small screen. There's been some efforts to build suites of mobile software, or UI toolkits that scale nicely, but it's behind the curve compared to the rest of the Linux ecosystem. Running any piece of Linux software might be nice in theory, but most probably don't really care about running desktop LibreOffice on their phone compared to just having an SMS app that works fine on a small screen. And then the more specific apps. While the best efforts in the world might make a suite of generic apps that work and look great on mobile, the problem is always going to be the third party apps that everyone's become used to. While some on this sub might be happy to avoid the likes of WhatsApp, Spotify, Discord, Uber, etc, a lot of people are too reliant on them to give them up. Web apps get some of the way there, but they usually lack something compared to a native app experience. For most people they're not going to give up those apps no matter how good an alternative is. At this point, I think for any other mobile OS to succeed, it either needs to support running Android apps out of the box, or somehow become so successful that major apps have to support it.