The reality of software products is that they are in nearly in all cases developed/maintained over time, though--and whenever that's the case, the black box metaphor fails. It's an idealization that only works for single moments of time, and yet software development typically extends through the entire period during which a product has users.
> I read OPs "good code" to mean "highly aesthetic code" (well laid out, good abstractions, good comments, etc. etc.)
The above is also why these properties you've mentioned shouldn't be considered aesthetic only: the software's likelihood of having tractable bugs, manageable performance concerns, or to adapt quickly to the demands of its users and the changing ecosystem it's embedded in are all affected by matters of abstraction selection, code organization, and documentation.
> I wouldn't care so long as I wasn't expected to maintain it.
But, if you’re the one putting out that software, of course you will have to maintain it! When your users come back with a bug or a “this flow is too slow,” you will have to wade into the innards (at least until AI can do that without mistakes).
And most users are not consuming your code. They’re consuming some compiled, transpiled, or minified version of it. But they do have expectations and it’s easier to amend the product if the source code is maintainable.
If it’s software that will never be modified, sure it doesn’t matter.
I would classify all of those as "capabilities and limitations of your product"
I read OPs "good code" to mean "highly aesthetic code" (well laid out, good abstractions, good comments, etc. etc.), and in that sense I agree no customer who's just using the product actually cares about that.
Another definition of "good code" is probably "code that meets the requirements without unexpected behavior" and in that sense of course end users care about good code, but you could give me two black boxes that act the same externally, one written as a single line , single character variables, etc. etc. etc. and another written to be readable, and I wouldn't care so long as I wasn't expected to maintain it.