C never was the lowest level of abstraction; there are other abstraction models out there and more still to be invented no doubt. C's model did well (though struggled mightily against Fortran for the longest time) at aligning with processor models of the 80's and 90's. It helps that C's ubiquity meant that to a degree the trail was washing the dog: processor designs were often measured against how they executed code compiled with a C compiler. But past preformance is a poor predictor of future success; who is to say that as processor designs continue to evolve, C's abstraction model won't become increasingly leaky against another? Absent a sufficiently smart compiler, it's entirely possible that C compiler writers will find themselves at a disadvantage.
And that assumes they're competing with a traditional compiler. It's possible, though unlikely, there will be competition from other execution models. As you said memory is often the core area of performance bottlenecks these days, and as terribly inefficient as bytecode interpreters are, they tend to have smaller code sizes. Efficient interprets are hand tuned specifically to make the overhead of bytecode interpretation as efficient as possible. Now, intrinsically they are performing a translation step at runtime that a compiler did before runtime, but one can at least theorize of a model where the interpreter is effectively a specialized decompression algorithm that feeds machine code to the CPU (that's really not that far afield from what happens in hardware in modern cpus and in mobile runtimes). Higher levels of abstraction might allow for more efficient decompression... It's crazy, but not inconceivable.
