Irreducible complexity and load-bearing bugs. Kludge built upon kludge built upon kludge. An engineer with the go ahead and all the support required would still struggle because the knock on effects of any one change cascade out in unpredictable ways. Not to mention working in an active environment where, although the other engineers support the goals in theory, they still need to deliver business requirements for a fickle management that doesn't truly understand what lurks beneath the facade - they don't have time to do it the right way, and if they try they'll just break both the old system and the new.