Although one key difference is that EE kept the IP for all the user generated content, but all SO content is licensed under Creative Commons.
If Stack Overflow decides to put up a paywall anyone could simply put up the all existing content, perhaps using one of the existing open source SO clones [1]. Also, a lot of the current Google traffic would instead go to one of the sites which currently legitimately repost all of SO's content [2].
[1] http://meta.stackoverflow.com/questions/2267/stack-overflow-... [2] http://meta.stackoverflow.com/questions/24611/is-it-legal-to...
For instance I don't mind ads that are relevant to me, like book recommendations. I also wouldn't mind complementary services related to Stack Overflow.
There are more options available right now than in the early 2000 that could earn them revenue without upsetting the community.
Instead of posting comments on the internet, you should be spending time with your loved ones.
If they are cynical, they might change license for new content after the community has gained enough traction. I'm not convinced people would stop using SO if they did.
So, license change -> major drop in participation as many will go elsewhere bringing with them SO's content up to that point -> might as well close the thing down, killing the golden goose and whatnot.
(btw, 1.there are worries that the community has already gained too much traction and the site's decline is imminent, as it'll surely be swamped with high-noise Q&A; and 2. speaking for myself, I'd surely quit if a restrictive license were put in place, there are many such users)