I don't think so. You see people don't have time now to watch test matches to their fullest like they used to do. Now they want to be entertained in the shortest possible duration. And what is better than a game in which a side has only 20 overs to play. The eleven players on each side have a much greater enthusiasm factor and too little time to worry about 'getting in' and 'staying on the pitch'. Everyone tries to hammer the ball from the word go. This is a really fast paced game where a good eye and a good day is essential. Batsmen are not expected to show their skills rather just whack the ball around. The better you can whack, the more entertaining you will be.

Now if talking about the future, batsmen and bowlers don't become great while playing T20 matches. The true players get to show their skills over long durations like in a 3 or 5 day match. Although demand of these are growing less by the day, they will never phase out because they are the venues where players improve themselves.

Attendance may get short, people may not watch them, but teams and coaches will never succumb only to T20 and will continue to play the longer versions of the game to be at the top.