I'm curious as to which other metric you'd use to define successful? If it actual usage, C still wins. Number of new lines pushed into production each year, or new project started, C is still high up the list, would be my guess.

Languages like Rust a probably more successful in terms of age vs. adoption speed. There's just a good number of platforms which aren't even supported, and where you have no other choice than C. Rust can't target most platforms, and it compiles on even less. Unless Linux want's to drop support for a good number of platforms, Rust adoption can only go so far.