The 11th, 12th, and 13th editions of the Encyclopedia Britannica were from 1911, 1922, and 1926.
So if, as a researcher, you were interested in the state of knowledge in 1916, you're out of luck (if you're limiting yourself to a particular encyclopedia, for example).
I don't know what value you see in "stability", when the world's knowledge changes on a second-to-second basis. It's equally arbitrary whether you freeze knowledge at 1911 or in any given point in time since Wikipedia began.
When you say "A Wikipedia snapshot proves nothing", I have no idea what that means. If you're worried about vandalism or quick edits, it's trivial enough to compare with earlier and later versions of the page to ensure you're looking at relatively stable text. Heck, most of that could even be automated if you wanted.
And while Wikipedia articles are full of errors, so too were the articles in every edition of Britannica. The difference is that Wikipedia errors tend to get fixed a lot quicker, while the Britannica errors just remained on the paper they were printed on.
I genuinely don't see what difference there is between the "stability" of the Britannica 1911 edition, and the "stability" of Wikipedia at some arbitrary timestamp. Both capture a similarly arbitrary moment in time -- Wikipedia just gives you so many more to choose from.