I'm even more bullish on computers assisting humans in memory allocation than ever. I think we as humans should just write programs and assume an unboundedly intelligent compiler will study our code, determine what if any lifetimes are clear in the code from usage (e.g. basically type-inferring ownership, rather than type-checking it), do trial region allocation, with dynamic profiling and adaptive optimization, and GC the rest with the best collector based on dynamic tuning.

Basically, just write assuming GC, then whereever a hint could possibly exist, throw computers at it. If anything, such hints should live outside the source code and have no semantic impact (i.e. program works fine if you throw it out).