Comment by epihelix

Because even if the rewrite worked, and worked perfectly, you've (a) lost a day, and (b) there's no guarantee that you won't have to do that all over again in a few days' time, because you're still not thinking about the architecture.

But if the codebase is large enough, and if you don't have tests for everything, you'll likely lose stuff along the way. So you'll then spend at least another day tidying up the rewrite, just getting it back to where you were two days ago.

Does that sound like fun? Wouldn't it be easier to just plan it correctly the first time?