A lot of things are like network protocols. Most things require communication. External APIs, existing data, familiar user interfaces, contracts, laws, etc.
Language itself (both formal and natural) depends on a shared understanding of terms, at least to some degree.
AI doesn't magically make the coordination and synchronisation overhead go away.
Also, reusing well debugged and battle tested code will always be far more reliable than recreating everything every time anything gets changed.
It could also be argued that "reuse" doesn't necessarily mean reusing the actual code as material, but reusing the concepts and algorithms. In that sense, most code is reuse of some previous code, written differently every time but expressing the same ideas, building on prior art and history.
That might support GP's comment that "code reuse" is overemphasized, since the code itself is not what's valuable, what the user wants is the computation it represents. If you can speak to a computer and get the same result, then no code is even necessary as a medium. (But internally, code is being generated on the fly.)
The point is that specifying and verifying requirements is a lot of work. It takes time and resources. This work has to be reused somehow.
We haven't found a way to precisely specify and verify requirements using only natural language. It requires formal language. Formal language that can be used by machines is called code.
So this is what leads me to the conclusion that we need some form of code reuse. But if we do have formal specifications, implementations can change and do not necessarily have to be reused. The question is why not.
Something like TLA+ model checking lets you verify that a protocol maintains safety invariants across all reachable states, regardless of who wrote the implementation. The hard part was always deciding what "correct" means in your specific domain.
Most teams skip formal specs because "we don't have time." If agents make implementations nearly free, that excuse disappears. The bottleneck shifts from writing code to defining correctness.
Typically people feel they're "forced" to use software for entirely valid reasons, such as said software being absolutely terrible to use. I'm sure that most people like using software that they feel like actually helps rather than hinders them.
And long term maintenance. If you use something. You have to maintain it. It's much better if someone else maintains it.
The whole idea of an OS is code reuse (and resources management). No need to setup the hardware to run your application. Then we have a lot of foundational subsystems like graphics, sound, input,... Crafting such subsystems and the associated libraries are hard and requires a lot of design thinking.
I mean it’s just software right? What value is there in reusing it if we can just write it ourselves?