Let's start from the obvious- Sandberg was very successful in AAA Lehigh Valley the last two years. In his first year, he took the team to the Governor's Cup Finals, where they lost 3-1 to Columbus. Arguably though, his second year was more impressive. Despite losing players such as Dom Brown, Kevin Fradsen, Jason Pridie, Erik Kratz, Pete Orr, Michael Martinez, and most of his bullpen at different points, but didn't get talents like Hernandez, Valle, and Pettibone until late, and never got Darin Ruf. With that said, he kept them in the race until the final days, and finished with the second winning record in franchise history (he was obviously the manager of the first). There's not much more for a Hall-of-Fame player, turned successful AAA Manager to prove. He's ready to advance his career. He's either got to sink or swim in the majors now. It's worth noting too, his performance as a third base coach is not necessarily important to the ultimate decision about him being the next manager. He's already proved he can manage a team.
And it's a manager we're going to need. Charlie Manuel is the most successful Phillies Manager ever, with five division titles, two NL titles, and a World crown, but he's in the final year of his deal. Someone has to replace him. Sandberg is certainly the right choice internally. Being that some of "his guys" are up now, it makes even more sense to see how he does here.
The decision to make this move was a good one for the Phillies. Ryne Sandberg isn't in question. He's the right guy to try now.