We'll have to see how thin that UI layer is, but I don't think the importance of the UI layer can be emphasized enough. At the very least, proprietary user interfaces and applications still carry all the risks of planned obsolescence, lack of user control/insight, and all the accompanying privacy and maintenance concerns.
Linux can be made to run acceptably on lots of devices, and there's plenty of unfunded community activity already going on that can deliver quite a bit of the stack - you just have to see how QtMoko (the community-maintained variant of Qtopia) still manages to roll along, regardless of whether anyone thinks it is blazing new trails - but cultivating a community around applications is something that the likes of Nokia only did in order to get access to the bits they needed. As a result of all this, people pin a lot of hope on Plasma Mobile (or whatever it's called this week) because it at least gives them the possibility to define their own platform.
I wouldn't be at all surprised if there's a similar level of enthusiasm for Jolla's "open core" as there was for Nokia's efforts, where contributors merely get the satisfaction that their code is being used from the outside of a tamper-proof glass case. The difference between then and now is hopefully that more people are wise to the game being played.