I don't think any of the three major formats will die. Far and away, the safest format in cricket today (like it or not) is 20/20 cricket. Most cricket countries have a competing domestic league which raises a few dollars, the sub-contenient love it and countries that are a bit off the pace standard-wise (West Indies) still have fans that are drawn to the 20/20 game. It is time friendly and the format that will draw the most new followers to cricket. It is here to stay.
Test cricket is the toughest format by far. It is so tough that only two countries seem to have a care about its long term future. Regardless, it will never die due to the top power countries tradition and will to keep it going. Test cricket will continue to have its challenges though.
ODI's will never die but will continue to get less attention as time goes on which is a shame. Im bloody enjoying this world cup at the moment as the 20/20 game has clearly added to the standard of ODI cricket.