I don't see why both OD and 20/20 can't live side by side, they are different sorts of cricket just like Tests and OD matches are there is no need to replace anything IMO.
I also agree with Neil that people's opinion of 20/20 just being a slogging match and will just develop the wrong sort of players isn't true and anyone who has seen the games will know that 20/20 has alot to offer the players as well.
- Improvised shot making, to try and find the gaps in the tight fields - Loads of players on the rope make boundary shots actually quite hard, and teams are happy to give away 6 singles.
- The above will help in the longer version of the game, in the last 15 overs etc where teams need to up the rate.
- Makes the bowlers try/improve things like yorkers/slower balls and bowl a better line and lenght.
- Improves the fielding of players, who know they can't give runs away and will go to extreme lenghts to prevent runs etc.
- 20/20 generally regardless of ablity will be close games, well at least the vast majority will go right to the wire hence improving the players ablities of performing under pressure.
- The time limits and rules, force bowlers to improve over rates - Something that is shocking in cricket at the momment, where I saw a team recent do something like just 12 in the hour... in 20/20 they have to do 20 in a hour and a half - If they can do it then, they can transfer that into the longer game.
- Captains get to learn as they can try new fields and experiment a bit - Thinsg they can then take into the longer games.
And that's just off the top of my head