In said hypothetical world, though, the whatever-driver would
also have been written by LLMs; and, if the printer or whatever is non-trivial and made by a typical large company, many LLM instances with a sizable amount of token spending over a long period of time.
So getting your own LLM rewrite to an equivalent point (or, rather, less buggy as that's the whole point!) would be rather expensive; at the absolute very least, certainly more expensive than if you still had the original source code to reference or modify (even if an LLM is the thing doing those). Having the original source code is still just strictly unconditionally better.
Never mind the question of how you even get your LLM to reverse-engineer & interact with & observe the physical hardware of your printer, and whatever wasted ink during debugging of the reinvention of what the original driver already did correctly.