It's fairly common knowledge that nearly every decision the ICC make is purely motivated by how much money they can make out of it. Scrapping a test championship to make room for a more lucrative Champions Trophy, booting the associates out of the World Cup so they can ensure the big guns make the final rounds (so they can sell the TV rights for more money), even a look towards day/night tests - the ICC isn't so much cricket's governing body as it is cricket's fundraising arm.
The ICC don't get many of their decisions right. The furore of getting rid of the smaller nations from the World Cups was so vociferous that eventually they had to backtrack - but only when the broadcasters threatened to pull out instead. Most of the tournaments are bloated, too long and not interesting - mainly due to the greed of getting as many matches in as possible in order to satisfy TV demand.
However, with the World T20, they have something right. A short, sharp tournament which lasts two weeks - just enough time for a plot to develop, but short enough for it to not drag on for a suicidally long time. The initial three-team group system means that every group game is important, meaning there's no dead rubbers, something which is a common feature of the seven-team group fifty over World Cup - plus the format is simple and easy to understand. Games happen thick and fast and teams don't have long from one game to the next, meaning momentum is built and interest is peaked. The World T20 is one hell of a tournament, and somehow, the ICC have stumbled across a winning formula.
However, with the ICC being the ICC, they now want to change that. Their proposal to increase the teams to fifteen (from the current twelve) would completely change the simple, quick and easy nature of the current set-up, with more games added, a far more complicated system implemented, and the excitement taken away. I know the ICC's move to add a couple more associates is done with noble intentions, but surely there must be a better way than to completely re-think the tournament itself? A qualifying stage that takes place immediately before the 'final' stages of the tournament perhaps? Or sticking all of the competing teams into a preliminary stage, before they move onto the current system as we know it?
In the World T20, the ICC have a golden egg - a tournament that really does work. So far, all three versions of it have been a massive success, and there's been no calls from any quarters for change. So why would they need to tinker? Instead of worrying about things that they're actually getting right, the ICC's time would be better spent on the far more pressing issues of the day, rather than turning the golden egg into yet another failed cash cow.