Are you sure? Perhaps it is just that a lot more code is being produced using other languages? Almost all of the software projects I follow and am interested in, are still written in C.
As an example, consider Brotli, the new LZ77 compression variant published recently by Google. Or the Linux kernel, and the rate at which it is being refactored and continuously rewritten, not just added to.
It is not like a typical desktop application actually solves any new problems, after all. The really interesting stuff is, and probably always has been, a small fraction of the overall produced code. Perhaps nowadays the total volume of new code is larger, and the interesting fraction smaller; it does not necessarily mean that the amount of interesting code written in C or C++ is declining or has declined. It might just get lost in the noise.
Yet, I don't see where programmers discuss possible solutions, or develop new ones. The end results just get posted to mailing lists, or appears in Git repositories.