When the Voyager 2 was made, it was with the intention that there won’t be any repairs, updates, nor changes to be done after launch.
It was a dead end product.
If there was ever any glitch, mistake, or malfunction, the only way to fix the situation is to launch another probe.
Dead end product development takes a huge amount of time during pre-frabrication; all use cases must be completely thought of and prepped, and all contingencies must be accounted for.
However, in some ways they are also much easier to design.
Since there is no expectation of change, a lot of efficiency “shortcuts” can be made. Power cells can be directly coupled to the modules that need power because those modules will never get reused. The outward appearance can contain many bits and pieces that make it ugly because: who cares?
Products like the Macbook or the iPhone are near the dead end products in the spectrum. They sacrifice modularity and reuse over space efficiency and the pursuit of thinness. They are almost impossible to properly repair and the best way to fix an iPhone is to get a new one.
There isn’t anything wrong with dead end products.
The problem comes with expecting reusability from them.