It's not likely to ever produce another massive category product hit like Stackoverflow. Such success requires very high levels of specialization and dedication (not to mention a category big enough to begin with; all of which are already dominated by very large competitors, eg cooking / recipes).

Why would cooking experts or financial pros be drawn to Stack Exchange? They won't be. They'll stick to sites dedicated to those things that possess a very narrow focus, where the quality of content is likely to be higher.

It was an interesting attempt on Stack's part to spread out, but who really thought the end result would be them taking over every category?

Just take a look at Personal Finance / Money. That's a huge category that is very lucrative. Yet they're getting a whopping 5 questions per day for a three year old site, and a mere 7,200 visits per day. Why? Because "money.stackexchange.com" can never be promoted properly as a brand, it can never carry any kind of reputation as a separate product. An average person isn't going to buy into the notion that that site possesses authority on personal financial matters.