> I want to form a mental model of what code I should write to get a certain effect
And how do you do that with hyperthreading, virtual memory, power management that may decide to power down your core because what you're doing doesn't seem important enough (and that differs greatly from one processor to another) and cache effects on code, data and TLB (all are strongly affected by other threads and processes running on your machine[1])?
While those effects didn't exist much before the 90s, and they don't exist today in GPUs and small embedded devices, on desktops and servers those effects may be much greater in magnitude than any difference you're able to get by better control over generated code. Not running a hypervisor, turning off virtual memory, pinning threads and isolating cores have a much more profound effect on predictability than which language or compiler you're using. Focusing on compilation before taking care of those much more powerful sources of unpredictability is like trying to get a faster car by reducing the weight of the upholstery fabric.
> so that a program that runs fine today might be unusably slow tomorrow.
I think that slowdown actually applies to assembly programs much more than to, say, Java. As CPU architecture changes, it's actually easier to keep higher-level code performant. I mean, why do you assume that compiler changes will hurt your code performance more than CPU changes?
> You can still have an LLVM-smart compiler underneath but you get to make the first pass.
There are many ways to produce good machine code (my favorite is Graal, HotSpot's next-gen JIT), but none of them really give you a good mental model of what's going on. You may like one approach over another for personal aesthetic reasons, one approach may actually produce better results for some workloads than others, and some approaches really are more predictable -- but no approach produces categorically predictable results, and more predictability doesn't buy you better performance (though it still requires more effort).
It used to be that if you knew what instructions your compiler would emit, you knew how your program would perform. That is just no longer the case (well, it is to some degree, but other effects are stronger). A single instruction may perform anywhere within 7 orders of magnitude (L1 cache hit to virtual memory miss) depending on effects outside the program's control! (of course, those high-volatility costs are usually amortized, but so is a less unpredictable compiler output).
[1]: That is the key to cryptographic attacks that let a process sense what a cryptography algorithm running in another process is doing by the way the cryptographic computation affects the performance of the first process.