The beginning of the end for the authoritative print encyclopedia was this 2005 Nature study, which found that in entries about science topics Wikipedia contained an average of 3.86 mistakes per article--but that Britannica contained 2.92 mistakes per article, putting the "free encyclopedia anyone can edit" within earlobe flicking distance of the shelf-bending gold standard.
(Come to think of it, the beginning of the end was when the price of an encyclopedia converged with the price of a computer you can carry in your pocket "holds" entire libraries of books, every website ever, and several competing encyclopedias. Today, a print set of the Encyclopedia Britannicacosts an order of magnitude more than an iPhone.)
This quote from today's New York Times coverage of the announcement is the encyclopedia equivalent of your Uncle Harry claiming that he's really hip and with it because he likes that new band, Radiohead:
The last print version is the 32-volume 2010 edition, which weighs 129 pounds and includes new entries on global warming and the Human Genome Project.
Citing Wikipedia as a source will still earn you a sneer in some quarters, but that's fading astonishingly fast. Some of the coverage of Britannica's shift to digital has been elegiac--heck, even I have warm and fuzzy feelings about the time I was doing a research project on the Minotaur and accidentally discovered that I am totally related to Genghis Khan--but the fact is that even if you're looking for informational serendipity (or convenience, or up to date information, or images, or further reading) Wikipedia has Britannica beat. We're so used to Wikipedia these days, that it's easy to forget what an utterly insane idea it was when the website was launched.
The NYT post mentions that several school libraries will continue to stock print encyclopedias, for kids "whose teachers require them to occasionally cite print sources, just to practice using print." Because when the apocalypse/Rapture/Mayan End Times comes and the power grid goes down, we might need to look up how to knap stone tools in the Macropedia (or is it the Micropedia? I never could keep them straight). Fair enough, actually. But when your best selling point is as a post-apocalyptic insurance plan, it's probably the right decision to call it a day.