I claim the following simple experiment supports this depressing claim. Run your favorite set of benchmarks with your favorite state-of-the-art optimizing compiler. Run the benchmarks both with and without optimizations enabled. The ratio of of those numbers represents the entirety of the contribution of compiler optimizations to speeding up those benchmarks. Let's assume that this ratio is about 4X for typical real-world applications, and let's further assume that compiler optimization work has been going on for about 36 years. These assumptions lead to the conclusion that compiler optimization advances double computing power every 18 years. QED.
This means that while hardware computing horsepower increases at roughly 60%/year, compiler optimizations contribute only 4%. Basically, compiler optimization work makes only marginal contributions.
Perhaps this means Programming Language Research should be concentrating on something other than optimizations. Perhaps programmer productivity is a more fruitful arena.