A team who allows messes to persist in their code will gradually slow down to a tiny fraction of their potential productivity. I’ve seen teams that have expanded their estimates out to weeks and months for jobs that ought to take a day or two.
The slower the team goes, the more the pressure builds. The more the pressure build the more messes are left uncleaned. Each such mess impedes the team even more; and the slower they all go. It’s a vicious cycle, and if you’ve been a programmer for more than a couple of years you’ve likely taken a few turns around it.
But it gets worse. Managers, desperate to increase productivity, hire new people. This, of course, has the opposite of the desired effect. The new people, thrown into the mass of messy code, see how things are done around here and — of course — they emulate it. So now there are even more people making messes, which drives the cycle even faster, and drags the productivity even lower.