CM Punk did not win the World Championship too early in 2008. WWE made the best decision that they could, given the situation. Punk got that push by defeating Shelton Benjamin, Chris Jericho, Carlito, MVP, Kennedy, Morrison, and Matt Hardy. The winner was SUPPOSED to be Jeff Hardy at the time, I recall hearing. That obviously could not have happened, so CM Punk was the best alternative out of who was left at the time so they put Matt Hardy in the match in his brother Jeff's place and made the decision to have Punk win it.
Who else in the match could have won and made it as a main eventer who actually needed it? Shelton Benjamin was never going to be main event material. Chris Jericho sure didn't need a Money In the Bank push and his heel turn that took place around the same time as Punk's cash-in breathed life back into his character in a way few saw coming. Carlito I won't even get started on. He shouldn't even have been IN the match. MVP perhaps but neither he nor Morrison were ready to main event. Matt Hardy was never going to get anywhere beyond a midcard title, and no the ECW Championship does not count as a world title. Kennedy blew it the previous year and never earned WWE's trust back, so not him. Who does that leave? CM Punk.
CM Punk was the only option left. He had the potential, and WWE had to put the title on someone at some point. Jeff could not win it like he supposedly was meant to and there had to be a MITB winner that year. This push helped Punk reach the potential he had always had. His cash-in was one of the most memorable Raw moments of all time, in particular I loved seeing Edge get a taste of his own medicine. His title reign wasn't bad either being put over by guys like Batista or JBL before losing it in a controversial way to Chris Jericho. Had Punk not done so well with this push then I doubt they would have given him a second MITB push the following year, which means no Straightedge Society push either. By that time he was ready to main event for real, even before the Pipe Bomb. Punk's 2008 World Championship reign was good for both him and the federation because they did the best that they could with what choices they had and pushed somebody new in the process.