From what I saw Cosmic has a lot of potential and looks pretty sleek too, right now I'm using KDE it's a great desktop, but now that I have a second monitor it randomly crashes on me, I think I'll switch to Cosmic when it reaches beta.
I think this rust only thing is gonna screw them on the long term. You really don't want that for app development, it might be a good choice for low level stuff and security sensitive things like browsers; but other than that you're severely hampering your contribution sources and increasing the development time. Color me skeptic but I see this going the same way unity did.