I think it should be possible to a working version of Maemo on a Samsung Galaxy or a Sony Xperia. There would certainly be missing functionality without the use of Android blobs and libhybris for those hardware components that require them, but I think a generic Linux operating system, like Maemo, should be bootable. Several Exynos and Snapdragon SoCs used in Samsung Galaxys and Sony Xperias already have mainline Linux support.

The use of kernels with an expiry date has been a major show stopper that has prevented me from purchasing an Android or even a Jolla phone. There are millions of Android devices with unpatched kernels still in use. Halium devices with Android kernels have the same problem. I don't think it will be long before we see another WannaCry-type worm aimed at unpatched EOL Android kernels with known vulnerabilities.