When I left C++ for managed languages, operating systems and C didn't stop being relevant; what stopped being relevant for me was the grotesque extensions of C dialect desperately trying to be a high level, managed language in increasingly irrational ways.
C++ is still the best tool when my managed languages need some extra native help.
Maybe it will be eventually replaced by Rust one day on my toolbox, but surely not for C.
I left that in 1993, only using it instead of C++ when university professors required me to do so, and on my first job (until they also moved into C++/C#).
As you mention lambdas take it to the next level, and in such future Linux might not even matter anymore, hence his point of view.