It's not just that the code is already written and debugged; in many cases results have been published using analysis done with this legacy software. Using the

It's not just that the code is already written and debugged; in many cases results have been published using analysis done with this legacy software. Using the same code ensures a certain amount of consistency between experiments from the same lab. From this point of view, rewriting code is an extremely high-risk proposition. As we know, the likelihood of discovering bugs during this process is quite high.