The problem with Legacy IT as a metaphor is quite simple: to the whole rest of the world, a legacy is a good thing. If you’re trying to get people to see something as bad, it’s really, really important not to hamper your mission by naming it after something good. We might as well call Legacy IT Fluffy Kitten and Unicorns IT for all the good it will do in rallying the (usually) financial troops to do something about the darn stuff.
Simon Wardley has suggested “Toxic IT” might be a better term. I think we should go the whole hog; let’s call it what it is. It’s Sh IT. No room for ambiguity there, thank you very much.