I would say if you rewrite it and have more bugs, you must have no understanding of the existing codebase, or understand so little of it you shouldn't be the on

I would say if you rewrite it and have more bugs, you must have no understanding of the existing codebase, or understand so little of it you shouldn't be the one rewriting it regardless. All the rewrites I have done have resulted in leaner codebases with fewer potentials for bugs. I think people take some article one software developer wrote almost two decades ago as gospel, but "second system syndrome" doesn't have to be a syndrome. This "rewrite" fear is exactly why our banks are stuck on 1970s and 80s code with an almost 60 year old programming language and no one left to take over maintenance. Eventually we need to get over this fear.