That a "working" prototype with no tests is just as good as a carefully-designed and well-tested feature. I see this happen so often that a coder puts a prototype in front of a product manager or exec and they are like, "this is exactly what we need, now! Ship that!" And then misery ensues for all of the engineers that need to maintain this piece of garbage. As managers pressure the engineers to build new features on top, they inevitably break fundamental parts of it, and without a confident leader to demand that tech debt is paid off, that product will consume the souls of many desperate coders.

In contrast, if you do it right the first time, there will be significant parts of code that never need to change, and the parts that do need to change will be much easier, because it will be obvious if it breaks the tests.