I almost preempted that counter-argument in my previous comment. :)

Some software development is 90% research and 10% development and it's true that you never really know how long it's going to take and how well it will work until it's almost done anyway. But the tar pits I'm talking about were not that kind of software development. Most of the cases I'm thinking of were the stereotype over-engineered "enterprise" code that had become bloated and excessively interconnected. Others were "clever" code where someone had tried some fancy design patterns or data structures or algorithms, typically with a severe YAGNI complex as well. Either way making simple changes required many times the effort it should. And yet a drop-in replacement for the whole system would have been a low risk project, with very predictable work required that could be done by any mid-senior developer on the relevant team, taking a fraction of the time.