Unfortunately, the only company that even conceivably has the resources to contemplate a "pure Linux" alternative to Android is Canonical/Ubuntu... and they dropped the ball so many times & made SO MANY design decisions that were just plain *awful* (like making literally EVERYTHING a gesture, to the complete exclusion of support for real hardware buttons AT ALL), their credibility in the mobile realm has been almost permanently ruined. And truth be told, even IF they pulled it off, they'd STILL basically be "Android, with a different wealthy corporate master". It's not like KDE or Gnome for Phones is coming anytime soon... if you have an Ubuntu phone, it's going to be running Unity. Guaranteed. And Unity sucks donkey balls (for the exact same reason that Windows 8 did: it sacrifices everything that makes PCs worthwhile at the holy altar of "write once, pretend to (sort of) limp everywhere, even if it means subjecting users to tedious and sub-optimal procedures for the sake of consistency across platforms).
It's sad, but sometimes, I really get nostalgic for Windows Mobile. WM6 finally got most of the core features right (and could be made sort of attractive with third-party apps and extensions), and WM7 was almost attractive out of the box. The fact is, Android was a cruel joke compared to Windows Mobile untill well into the Froyo era, and got most of its popularity JUST because Microsoft decided Windows Mobile should go away more than a year before its replacement was anywhere close to being ready, let alone its worthy successor. IMHO, if Microsoft had refrained from totally fucking up their mobile platform & just kept evolving Windows Mobile forward, it would probably have at LEAST half the market currently owned (pwn3e?) by Android.
OK, sure... you couldn't build Windows Mobile from source. But then again, even a NEXUS device whose ROM is 100% built from source has deficiencies compared to one running Google's very non-open ROMs. Don't believe me? Try using Google Wallet with an AOSP-based ROM on a Nexus 6p. The last time I checked, AOSP ROMs couldn't use the secure element of the NFC chip, so no NFC payments for you if you dared to build your own ROM. Fuck, even CYANOGEN went proprietary with the One+ One (it technically had a ROM developed BY Cyanogen, but end users couldn't independently build it from source or modify it... it was all or nothing: use the closed Cyanogen binary as delivered, or settle for a less-capable AOSP-derived ROM that might have been based on generic Cyanogen, but lacked features specific to the One+ One's Cyanogen-derived ROM. At the end of the day, Windows Mobile (5 and 6, at least) was at least as upgradable by end users as a Samsung Android phone... possibly MORE. Ten years ago, we were ripping .exe and .dll files from newer phones and copying them to the filesystems of older ones. Today, we often end up doing the EXACT SAME FUCKING THING with Android phones whose manufacturer has all but abandoned them. The main difference? Every new Linux kernel catastrophically breaks every loadable kernel module that came before it, while Windows Mobile (like Windows desktop) had a fairly robust Hardware Abstraction Layer (with a bit of tweaking limited to editing .inf files and possibly signing the driver with a private cert, even Windows 10 can often be coaxed and teased into loading drivers built for Windows NT 4).
Yeah, I've gotten cynical. Five years ago, I eagerly looked forward to the day when I could download everything needed to build the exact same ROM provided by the manufacturer, then use it as a starting point to make it work the way **I** wanted it to work. The harsh reality is that AOSP has stumbled badly with non-Nexus hardware over the past 2-3 years or so (compared to its golden age circa the Galaxy S3, to the point where Cyanogen officially quit supporting Samsung devices, and other manufacturers might as well have, because supporting chipsets with no real public documentation [like Exynos and Qualcomm's] is damn near impossible), and even Nexus devices running AOSP-derived ROMs are missing functionality provided by Google's proprietary ROMs.
Five years ago, a new phone meant a gigantic leap forward... from 320x240 to 480x320 to 854x600 to 1280x720/800 to 1920x1080/1200 to 2160x1600, from 512 megs of flash up to hundreds of gigabytes, and from anemic single-core CPUs wheezing at 308MHz to 8-core CPUs with specs that would have been impressive for a Windows-running LAPTOP just a few years ago. Now, a brand new 2016 model phone is basically the 2015 model with cheaper hardware that has fewer features... and is probably missing the headphone jack, so instead you'll have to carry around an octopus cable and mini-hub to simultaneously connect all the things you USED to be able to connect to the phone itself.