Popularity has nothing to do with it. Look at sports like Baseball and American football. Leagues like MLB and NFL are among the richest in the world. Let's have a 4-month window for the IPL, so that it generates the maximum revenues. The remaining 8 months can be provided to other franchise leagues, ICC tournaments and bilateral tours. At this point, the IPL is much more profitable than any of the other options. So ideally the ICC should allow it to expand. I would suggest a 20-team tournament spanning over 4 months.
In my view the success of a tournament depends upon the quality of the matches played and how long it can attract the fans to tune in and how long. If the tournament becomes bigger, you cannot expect the viewers sticking through all the matches and it is all connected to the revenue and it is directly linked to the advertisement the media gets during the match. If the viewers goes down it will affect the revenue market as well. If media channels are not getting the advertisement they expect they wont be shelling out huge amounts for broadcasting rights.