It is very important to consider why the code turned into such a mess to begin with. Chances are same issues will happen again with the new codebase. If code generally turns worse over time, then the problem is not the actual code.

Developers are quick to think "some idiot must have written that" just because code is convoluted and ugly. But it might as well be smart competent developers working in firefighter mode, developers being shifted around from project to project with not enough time to get acquainted with the codebase, and so on. If these are the reasons a rewrite will not help.