It's probably the same as that which was missing from C++ in, say, 1990 -- the needs (and fashions) of tomorrow. But it's not specifically Rust. All languages are built for the needs of their time, and then have to adapt and change as requirements and fashions change. The question is, how do they adapt? That's defined by the language's ethos. C, C++, Java, Python, and C# have all survived and adapted for some time, but they've had somewhat different adaptation approaches. Over time, their domain also shifted (in fact, contracted compared to its peak; Python is exceptional, perhaps, in that it's peak domain wasn't its initial domain).