You seem to make some assumptions, but I'm not sure there's any basis for them.

I think no one disagrees. Still though, just like on desktop, if you want to run Liunx with a DE on older hardware you'd probably best go for XFCE or LXDE/QT or Mate. Plasma (mobile) might not be the way for you (or just not the latest version).

I don't exactly see how this connects with Plasma mobile though. The reason that the pre-built image for the Nexus 5 is broken is not because the phone or support for it has been abandoned. On the contrary.

Time, or rather, the lack of it. As far as I know, there are only a few 'main' devs actually working on Plasma mobile core parts such as kernel and boot/root image development. At this alpha stage of development with all the improvements and patched being made I understand it's hard for them to keep all pre-built images up and running for all different devices.

By the way, the post on the KDE forum you're linking to in your edit is posted by me As I stated there:

The workaround I posted just helps to get a very old development image running (built 24th of may).

All in all, perhaps you are judging a bit too soon. At this moment I'm attempting to rebuild the Nexus 5 image myself. I own one and would also really to get a recent version of Plasma mobile running on it. A user in the Plasma mobile Telegram group pointed me to some kernel patches which should make the current development version booting on it again. In case I'm successful, I'll be sure to let you know (don't hold your breath though )