This is the common argument from proponents of compiler autovectorization. An example like what you have is very simple, so modern compilers would turn it into SIMD code without a problem.

In practice, though, the cases that compilers can successfully autovectorize are very limited relative to the total problem space that SIMD is solving. Plus, if I rely on that, it leaves me vulnerable to regressions in the compiler vectorizer.

Ultimately for me, I would rather write the implementation myself and know what is being generated versus trying to write high-level code in just the right way to make the compiler generate what I want.