Just the pure satisfaction of knowing that I totally know 1001 ways to do those things more effectively and I'm going to turn this edge into cash. Simple.
Can you give me a concrete example of what you mean. A lot of things that people claim as being "legacy" are service/server level (i.e. a database engine, a runtime, a programming language, a protocol, etc) - these don't have any intrinsic UX or UI so I'm not sure I understand what type of things you're actually referring to
What some people call "legacy", others call "reliable", "battle tested", etc.