It's not sunken cost. Vala is performant, plays well with a multitude of languages, doesn't require a VM, and makes it easy to write Gtk applications. Unlike Rust it's very simple to learn and the elementaryOS project likes it for that reason since it lowers the barrier to entry for app developers. Based on this I'd say it's still got a lot of things going for it.
Now is there more to improve? Absolutely. Despite its use by users, development of the language itself suffers from chronic underinvestment. But I think that's changing a bit recently. (Full disclosure: I'm involved in developing the language and tooling. We could always use more contributors.)
Now is there more to improve? Absolutely. Despite its use by users, development of the language itself suffers from chronic underinvestment. But I think that's changing a bit recently. (Full disclosure: I'm involved in developing the language and tooling. We could always use more contributors.)