I'm super excited for doing mobile app development in something that doesn't require Android's overly complex toolchains.
I'm fine with letting the platform mature a little before the scope of the project is broadened.
But it was in the prototype stage then.
I use micropython in several of my projects for CPUs that are waaaay less powered than the thing they're putting in the Librem5 and, despite not being a 100% feature full implementation of python, it's still a very capable platform. Granted, I don't drive any UIs (yet), but those components would be handled by some native libraries anyways on the Librem5, and the choice of python vs C would not matter from that respect.
From my experience with Vala (compiles to C) it would have been a much better choice.
As for rust, I'd prefer they get a stable ABI so my phone doesn't end up with statically compiled bloat.
So as soon as I can access a Libre-phone, I'll put my favourite Lua-based engines on this thing, work out a toll-free-bridge for any of the nice OS things, and put all my apps on it as a target platform.
Just a matter of having the hardware.
(That said, there is absolutely no reason one could not do the same thing: use your language-of-choice, with a bridging layer. The use of C makes this a lot more tenable than, say, other systems languages.. at least in terms of tooling and methodology.)