What could C/C++ "lose" if they defined a standard ABI?

The freedom to implement things in the most natural way on each processor.

I imagine that c in particular has conforming implementations on more different architectures than any other language. Abiding by a ABI optimized for the currently common, high-end, general-purpose CPUs would require unnatural contortions on some the odder machines out there.

Backwards compatibility on every platform except for the one whose ABI was chosen.

Rather than a generic ABI for all platforms (which would be disastrous as it would only be optimal for only one platform). The standard's committee could say that each platform will conform to a specific ABI.

But: Who defines it (the first compiler through the door?). In which case they get an excessive competitive advantage. Or a committee after 5 years of compilers (which would be another horrible idea).

Also it does not give the compiler leaway to do further research into new optimization strategies, you would be stuck with the tricks available at the point where the standard was defined.

Execution speed would suffer drastically on a majority of platforms. So much so that it would likely no longer be reasonable to use the C language for a number of embedded platforms. The standards body could be liable for an antitrust suit brought by the makers of the various chips not compatible with the ABI.

Well, there wouldn't be one standard ABI, but about 1000. You would need one for every combination of OS and processor architecture.

Initially, nothing would be lost. But eventually, somebody would find some horrible bug and they would either fix it, breaking the ABI, or leave it, causing problems.

I think that the situation right now is fine. Any OS is free to define an ABI for itself (and they do), which makes sense. It should be the job of the OS to define its ABI, not the C/C++ standard.