That’s not how this goes.

Because the entire codebase is crap, each user encounters a different bug. So now all your customers are mad, but they’re all mad for different reasons, and support is powerless to do anything about it. The problems pile up but they’re can’t be solved without a competent rewrite. This is a bad place to be.

And at some level of sloppiness you can get load bearing bugs, where there’s an unknown amount of behavior that’s dependent on core logic being dead wrong. Yes, I’ve encountered that one…

If you can see the future and know no-one will ever encounter it, maybe not. But in the real world you presumably think there's some risk (unless no-one is using this codebase at all - but in that case the whole thing has negative business value, since it's incurring some cost and providing no benefit).