Maemo phones did not isolate the cellular modem from the rest of the system. Also, a number of essential packages on the Nokia N900 were (and still remain today) closed source.

It's odd that Jolla doesn't just open-source Sailfish OS.

Jolla doesn’t make devices any more. Their only business case is licensing their OS to paying customers. While many packages in Sailfish are open-source, naturally the company is holding back a lot of things in order to serve as the exclusive provider of the OS.

Weren't a lot of the useful components for Maemo open source? Eg: the GTK+ Hildon stuff for example? Why can't/didn't Purism re-use any of that?

A number of under the hood elements are being reused. Telepathy still drives the messaging system, and ofono is being used for the modem. The GTK2 UI from Maemo was dead in the water even before the shift to QT when Meego was released, so it makes sense that they would base a new GTK UI on a modern Gnome. A QT UI is available in the form of SailfishOS.

Regardless, the "interesting" work on the Librem 5 sits with none of these levels, but further down the stack, with mainlining the kernel work needed for the device. After that, the userland work provided by Gnome/Plasma Mobile/Ubuntu Touch/Mer all become interchangeable.

Sorry off topic but if I could follow one person for experience, not bring biased and knowledge of the state of the art of software tooling (and tonnes of other things... but tools are useful and not easy to find) then I would follow you. Your comments are very informative.

Thanks, just a random grey beard guy with too much free time between builds. :)