A rewrite is likely to take a lot of time, and if you're not careful, it's easy to end up running two systems rather than one at the end: the new one that doesn't quite do everything, and the old system that still does some important things.
In addition, if you don't change the development conditions, you're likely to end up with a similar mess at the end. Sometimes, code is messy because you didn't know what you were doing when you started and a rewrite could help; but sometimes code is messy because the requirements are messy and change rather a lot --- a rewrite can't help much with that.
That doesn't mean never do a rewrite, but you've got to have a pretty good reason, and it sure helps to have an incremental plan so that you don't end up with two systems and so that you start seeing the fruits of your labor quickly.
In addition, if you don't change the development conditions, you're likely to end up with a similar mess at the end. Sometimes, code is messy because you didn't know what you were doing when you started and a rewrite could help; but sometimes code is messy because the requirements are messy and change rather a lot --- a rewrite can't help much with that.
That doesn't mean never do a rewrite, but you've got to have a pretty good reason, and it sure helps to have an incremental plan so that you don't end up with two systems and so that you start seeing the fruits of your labor quickly.