This isn't what happened. When Vala came out (2006), it was at the time the only programming language advertising such high-level concepts as found in C# (like async/await) while guaranteeing you native performance without a VM. At the time it was billed as a better C++ and a faster C#. When the elementary project adopted Vala (2007), Swift and Rust weren't things, and they would remain unstable for at least the next decade.
Today the world has changed, and while I'd argue that Rust is one of the best languages invented, Vala still has some things over it: easier to learn, binds well with C and a million other languages, faster compilation and a simpler toolchain, has built-in type annotations for Gtk and DBus, all while guaranteeing you native performance.