But that isn't taking into effect the contagion issue. In theory your view is the way it works. In practice doing it "properly" comes out half-properly because you spent all your other time maintaining horrible code so that it infects your thought processes and design.

The problem is this. Let me put it clearly:

If you are maintaining bad code, it will take you several tries to come up with a good architecture to replace it. You will make better progress having someone entirely outside do the architecture themselves.

OTOH, if employers are going to use credit scores to decide whether to hire you, then technical debt may be an even better metaphor if you factor that in ;-)