I've been lucky enough in the general case to either be in charge of system design or have very thoughtful coworkers who are well aware of the burden of technical debt.

Unfortunately I can't babysit every other team and have occasionally inherited some real abominations. Often times it has been worth spending the ~week in man hours to immediately rewrite them to avoid the constant time suck of maintaining spaghetti code down the line.

Often they are a kernel of simple functionality buried in thousands of lines of code smeared across a random assortment of files that can actually be replaced fairly simply once you've sussed out what they're actually supposed to be doing.