This reminds me of a text editor I've seen linked here a while ago; it was written by one guy, and he'd been selling it and maintaining it for something like 30 years (and he still got new customers!). I don't remember the name of the program or the website.
Also, Bob Staake, a New Yorker illustrator, has been doing all his work in Photoshop 3.0 until 2008, and he's probably still using it. He was running it in the Classic environment on a PPC mac in 2008, I remember reading that he switched to a VM recently but can't find any info on it.