On a typical app, the user will have to trade off: peak/average memory, peak/average cpu, file storage usage, network, latency, throughput, cache behaviour, tolerance to pauses, and somehow produce a viable weighting function out of all of this.

This function will need a tonne of refinement, because the initial weighting or three will lead to wastage, and it will end up being complex and nonlinear. The optimisation process will be expensive and time-consuming.

The tuning process will be basically voodoo, harder than easily documentable configurations. Right now, experience can lead to dramatic improvements with ease. At the moment, tuning GC will also give you pointers for how to improve your program; and tuning your program will also give you pointers for tuning GC. That'll go.

Then, we get the same problems as before wrt contention for system resources, so the system has to somehow be aware of all the other programs running on the computer.

This all ends up being very difficult; much harder than performance tuning.

Again, things that the system can work out on its own, the user shouldn't specify. Things that the system can't work out on its own, the user should specify, or the system should make a good guess.