Rust

Yes, Fortran is still used heavily in certain domains, but no longer because it is, as it once was, the only reasonable choice. Nowadays its continued use is more a matter of inertia than of technical merit.

You seem to be asserting that C++ will always be the only reasonable choice for those difficult problems where there is, at this moment, no other. People used to say that about Fortran. They were wrong too.

That's not to say that Rust (or its descendant, TiN?) really will be the new heavy. What is certain is that there will someday be a new heavy that is not C++XX. Furthermore, it will differ from C++ in many of the ways that Rust does.