Also, this theoretical c compiler could make use of implicit hints (like which variables to place together in memory), making it faster than JS (except asymptotically, when the JIT has run long enough to figure everything out).

But the original point is basically right - one day JITs will be faster.

JITs don't have so much time to optimize, as static compilers do. Therefore, they can't do e.g. whole program analysis, or expensive cache related optimizations. Despite 15 years of effort of speeding up JVM, it is still slower in general than optimized C. Not by much (usually good Java code runs within a 2x slower margin), but still - it can't beat static compilers.