The nice thing about the way Minecraft has been implemented is that the login system is pretty much irrelevant to playing the game. They didn't cram it full of useless DRM, so one day when the auth servers are gone it won't matter as much to distribute a simple launcher or launcher patch that lets people type in what ever name they want to play with. It's probably how the pirated versions work already, if I had to guess. Servers let you disable the auth check in the config so those would live on as well. Unlike Minecraft, there's a lot of other games we've forked over money for over the years but they'll be gone forever one day when there's no way to log in to play/download them.
The one possible downside to Minecraft's longevity though are the libraries and assets. I saw someone distributing a script after the Microsoft buyout that grabs every version JAR from their servers, "just in case", but that's not enough, especially for modern versions. For 1.6+, you need the json file to know the libraries to fetch and the assets index, and then you need the assets json based on that index, from which you can determine the assets to fetch. Probably wouldn't even be a bad idea to write a script to fetch all of this for each version and archive it, "just in case."
