I think we've primarily succeeded in making nice Free Software desktop environments for _ourselves_. Maybe just having it available to those who are interested is a sufficient achievement.
Look at pop music, television, Hollywood movies, and the most popular "restaurant" in the world (McDonald's), and then consider whether the general public is ever going to care what computer operating system is technically superior. That isn't going to happen. We are never going to see the general public messing with partition tables to install another OS.
What we could see is Windows becoming irrelevant, with OEMs then saying "MS stuff costs money, but Android/Linux/whatever is free, and customers don't care anymore, so we are going with what's free".
Anyway, even if "desktop linux" never goes beyond the 1% mark, that doesn't make it a "massive failure" in my book.