I'm pretty sure this entire thread is filled with "nobody likes to maintain the pile of ifs", since I doubt very seriously it's the age that jams people up, it's finding the correct place to make a surgical change that only produces the net-new behavior without blowing up the world. I guess the rest of that is that often the older a codebase is, the more revenue stream in impacts if something goes wrong
I've very much enjoyed maintaining or optimizing or hardening existing systems--I can just never convince my leadership to let me do that.
My current org has a terrible case of not-invented-here syndrome, and it's so easy to pitch new projects that solve something that there's already an existing tool for, or building a new feature. We would love to spend time just working within our existing systems and fixing crap abstractions we made under the deadline-gun, but we're not "allowed" to.
> [...] humans like to build the new, and nobody likes to maintain the old
I think this is certainly true at organizational scale, but most of the people I've met are change-resistant overall.