The Android platform represents an enormous amount of work that encompasses a more secure base kernel, an unparalleled selection of applications designed for mobile usage and written in a memory-safe language, fantastic sandboxing and user privacy features leagues ahead of any desktop operating system, and great diversity in the hardware market.
While it's apparent that Android has significant downsides - de facto proprietary drivers, an environment of mostly closed-source applications, some Big G integration, and a general lack of long-term updates occasionally countering the work put into security (thanks, chip manufacturers: https://news.ycombinator.com/item?id=26593274), these are problems that can be solved. It doesn't make sense to throw the baby out with the bathwater.
Closed-source applications are more a moral problem than a security one thanks to Android's sandboxing, and a vibrant ecosystem of high-quality free-and-open-source Android applications (https://f-droid.org - Amaze, Notally, QKSMS, and Tasks are some of my favorites) makes them all but optional. Google's presence in the AOSP codebase is rather limited (mostly around notifications) and taken out altogether by custom ROMs like Lineage and GrapheneOS. Proprietary drivers remain as Android's biggest problem, perhaps alongside the manufacturers that ship locked-down phones with bloatware (looking at you, Samsung).
Why go the desktop-Linux-to-mobile route when you could fork Android, write drivers for the Pinephone / Librem 5, punch a hole to the base system for privileged applications, and have the best of both worlds with an order of magnitude less effort?