Once upon a time all three formats of cricket were very popular but a lot of things can be lost with the passage of time. Only that will be survived which are loved by current or future generations. In T20 format a game can be completed with in very short of time. Which is not possible in other formats. There are also some peculiarities. Time has a big contribution to make the game of football so popular today. Here the game ends in 90 minutes. Test cricket or ODI requires a whole day or more. Considering the overall aspect, everyone's eyes are on the T20 format of cricket.
I don't know whether we can make such conclusions. T20 never peacefully co-existed with other formats. Even the fanbase for T20 is different from other formats. A large part of the T20 fans were previously not interested in cricket. And the new generation is more used to T20 cricket and they have hardly any interest in the longer formats. It is mostly the middle-aged and the elderly who are remaining as the fans for ODI and test formats. Test format is tailor made for pensioners, who don't know how to spend their time.
as you know, the T20 format is a very exciting one and it is very fast-paced at the same time, there is no doubt that it is because of this that so many people who were previously not interested in cricket are now showing a healthy amount of enthusiasm for it, especially in the T20 format
moreover, i believe that with the introduction of the T20 format and the introduction of the T10 format very recently, i believe that cricket will be able to gain popularity once again among a wide range of people once the ICC concentrates on these shorter formats
even though Test matches and One-Day Internationals are still very much in demand by a very small percentage of people in the world, i think that the ICC should concentrate more on the shorter formats from now on instead of the longer ones