I have no example of a skill that took "a lifetime to master" as I am only 28 years old and would not call myself a "master" of anything at this point. However, I can describe the last two projects I worked on.
One was implenting an image quality algorithm for a medical imaging system. I did not do this on my own, but I will wager that it was far more complex and involved than any UI code you last wrote. I had to understand the science behind the algorithm, implement it, and then make it fast (it is run on huge images).
My next project was optimizing a heap memory manager for the device which captures these images. Not PhD level physics here, but not trivial and not something I would expect a UX guy to do.
So yeah, I take a bit of offense when you lump all programmers into the same bucket. Not all programming is engineering IMO, and to compare writing a UI to writing a thread scheduler, a memory manager, a new programming language, etc. is just ridiculous.
I'm not saying that UI work is dumb simple. It is incredibly hard to get right, but that kind of complexity is not technical. It takes a certain kind of aptitude and knowledge to do work that is closer to the metal and closer to concepts taught in CS. Creating a UI is not CS, it is wiring up bits of code that other people wrote in a such a way to be useful to a non-programmer.
So yes, a UX guy could probably be writing his or her own UI's in about 6 months to a year. I would not hire this person into the group I work on because they will be overwhelmed by technical complexity that they simply won't be able to handle.
