I don't know. Three expansions seems like a lot already. At a certain point there's a diminishing returns thing going. Adding more stuff to a game becomes harder without unbalancing it or detracting from its core strengths.
It's like the stereotype of American television series vs. British ones: the British ones often only have a dozen episodes and you wish there were more. The American ones continue for two seasons after you wish they'd stopped. You want to hit the middle ground between that, and 3 expansions seems like it should be plenty for that.
Now I get wanting more content and variety even after that, but that's why we've got modding tools. AoW got some great ones, and I know for a fact there are good modders waiting until DLC development STOPS before starting to create mods.
The advantage of mods over endless DLCs is that you can pick and choose which you want to use, and even change them from game to game, whereas DLC and patches tend to change the game permanently and retroactively, to the point where playing the original campaigns now is a different experience than when they were released.