If this is true, then at some point we will stop committing source code to repositories, and instead commit prompts. The LLM would be a "compiler" that can reliably turn a vast collection of natural-language prompts into a complete system.

I just don't see that, not least of all because natural language is inherently ambiguous, whereas all the other rungs in your latter ("machine code -> assembly -> C/JVM -> some lang") are completely unambiguous by design. Consider "I saw the man with the binoculars". Does that mean "I used binoculars to look at the man", or "The man I looked at was holding binoculars"? This is the kind of inherent ambiguity that Lojban was invented to mitigate. Maybe some day we'll write "natural" language prompts in Lojban that can be unambiguously translated by an LLM, but that sounds a lot like just using a "some lang".