Of all the major operational risks our startup faces, that would basically be at the absolute top of the list. I suspect we'd have to double headcount if SO shut down! SO should be considered a national utility/treasure like DTCC, etc.
Not just your startup. A large part of the tech industry would collectively panic if Stack Overflow shut down and all those answers vanished from Google. Developer productivity would likely be cut in half (or more) that day!
Lull in productivity, bad developers would suddenly stick out and either learn to "learn" or be replaced and productivity/quality would improve. It's all prospective :)
But I can't shake off the feeling that the company hired too fast and decided to let go of a few folks to avoid raising more funding. Of course as you said, the whole thing was amicable.