There's no value in SO's database of questions and answers, they might as well give it away under whatever liberal license they want.

The real business value of SO is the engaged ("community") web traffic it receives. ("Community" == "higher ad CPM" and "highly contextual advertising" in business terms.)

That community would become rather less engaged, and thus less valuable, if a paywall were implemented.

The user you are talking to is "qwe123_troll"

qwe123[4] is a troll account name with a storied history: http://www.reddit.com/user/qwe1234

Implementing a paywall would be rather pointless, since that's a very bad way to monetize traffic.

However, StackOverflow can make big money by cleverly partnering with corporations: instead of a site that answers "how do I do XYZ with ABC toolkit" they could gradually shift to a site that answers "doing XYZ with BigCorp ExpensiveSolution: how I increased ROI and fixed my dental problems".

The point is that between "comunity" and "traffic monetization" there is a clear and obvious conflict of interest, and it's pretty obvious which side of this conflict will be taken by SO's sponsors.

Incidentally, this is also exactly why Wikipedia doesn't display ads or monetize traffic.