There have been many contenders, notably Eiffel, Oberon and D. There are many others that I don't immediately remember the names of.
These languages have /great/ technical merit. They offer safety (in various forms) and other interesting technologies that C definitely lacks. They were lauded by academics and industry pundits. So why didn't they take the industry by storm?
Perceived technical merit is a terrible way to choose a language.
Pascal was widely regarded as a great language, a wonderful model, and it was widely used in the 80s by various large companies. Today it is mostly dead. I believe this is because Pascal only did an adequate job of expressing stuff at the hardware and kernel level, and that C was better. Certainly nearly everyone at Apple that I worked with breathed a sigh of relief when it became obvious that it was okay to write C instead of Pascal for new projects. For the most part we'd been writing C for years anyway, just in Pascal. About the only thing that people missed were nested procedures (whereupon, C++).
Your new "adult" language is going to need a set of very compelling offerings over and above "well, it's safer" in order to succeed.
Take a look at things people are doing /to/ C in order to be better:
- "analyze" builds that do control graph analysis and find bugs (not just ones endemic to C, but actual logic bugs, too)
- declarative sugar that helps tools to reason about what things like drivers are trying to do
- control extensions (commonly seen as macros providing 'foreach' like support)
- ways for tools to enforce local conventions (without spending tons of manpower on parsers and so forth)
Come up with a language as good as C at low-level programming, that has great debugger support, offers easy tool plugins, and that has interoperability with the gazillions of libraries already available [take a page from C#'s great interop story here], and you might have something. Go "academic" and just say "this is good for you, use it instead," and the working programmers will see nothing in it for them and ignore you, just as they've ignored or abandoned dozens of other offerings in the last 30 years.