You're right that flat datastructures are important, but C is far from the only language that can offer those.

I don't think C can ever be the answer; "comparable C code", i.e. code that was produced with the same amount of developer effort, is almost always undefined behaviour that happens to work most of the time on the developer's compiler. C doesn't have the kind of polymorphism that you need to write code that's compositional, testable, but also has good cache locality, at least not for code that also has to interact back and forth with the outside world. There is at least some work in the literature on making efficient use of cache in a Haskell/ML-like evaluation-by-reduction model, and I think that's where the long-term way forward lies (and FWIW in my experience compilers for ML-family languages already more than smart enough).