When it comes time to rewrite, it's usually because you've got an application which was successful in satisfying the users/customers needs, and which most often

When it comes time to rewrite, it's usually because you've got an application which was successful in satisfying the users/customers needs, and which most often reached that point going through lots of iterations, changes, trials and errors - accumulating technical debt in the process. At that point you've got pretty clear specs: what the original application does. Rewriting in a language which makes exploration slower, but long term maintainability easier, makes a lot of sense. This doesn't imply that using the latter language during the exploratory phase would have gotten better - or even the same - results.