Any team with as much losing in its history as the Phillies is bound to have had some unsuccessful managers. Click to the Franchise Encyclopedia page on B-R and the names jump out at you: Nick Leyva (1989-91, 148-189, .438 winning percentage), Frank Lucchesi (1970-72, 166-233, .416), Ben Chapman (1945-48, 196-276, .415 and a racist bastard to boot), Doc Prothro (1939-41; 138-320, .301). There are a couple others who didn't fare so well helming the Phillies--Terry Francona, arguably Gene Mauch--but had success elsewhere, and still others whose won-lost numbers weren't necessarily bad but nonetheless weren't well regarded: it amazes me that John Felske, whom I've always thought of as beau ideal of the feckless Phillies manager, was only four games under .500 for his tenure (1985-87). I can't speak for the nearly 90 years before Danny Ozark, the first Phillies manager of my fandom. But in my lifetime, they've never had one--even Felske--who was as clearly inept at the job as Sandberg.
It actually wasn't hard to see the logic in the hire. Beyond his Hall of Fame credentials for the work he did as a player, Sandberg had worked his way up the minor league chain, showing devotion and hunger for work in the dugout unusual for a guy who'd been a superstar on the field. (By contrast, Phillies legend Mike Schmidt spent one year in charge of the high-A Clearwater Threshers in 2004, went 55-81 and concluded managing wasn't for him.) The Cubs had passed him over twice, but that was easily enough explained--a reluctance to put a legend in charge of a bad team--even if you credited their front office with particular insight. His accomplishments as a player seemed to insure the veterans' respect, and his success in the minors suggested he could both win and develop talent--a key balance for the transitioning Phillies of 2013 and beyond.
Over his first season-plus, Sandberg gave little reason to believe he could be the bridge to the next crop of good Phillies teams. Nobody really over performed, and he had run-ins with veterans like Jimmy Rollins as well as younger players like David Buchanan and Domonic Brown. (We'll give him a pass on Kyle Kendrick.) But in 2015, Sandberg seemed to cross over into active harm. Admittedly he outperformed his Pythagorean record--but I'm almost sure that was mostly because he turned a lot of potential 5-2 losses into 10-2 embarrassments.
He blended tactical ineptitude with seeming inattention to player usage; as multiple bloggers have pointed out, Sandberg's resignation might have saved him from a future grizzly comeuppance at the hands of Justin DeFratus. But his worst failing, in particularly sharp contrast with generally loved predecessor Charlie Manuel, had to be in managing the clubhouse. Where Larry Bowa--Ryno's bench coach and wingman, still on the job as of this writing but thankfully not the interim skipper--at least had the respect of a few of the players who also hated him during his 2001-2004 managerial tenure, it's pretty clear nobody in that clubhouse is mourning Sandberg's departure, and likely that at least a handful (Ken Giles, Brown, even Chase Utley) are mouthing covert woo-hoos. That he didn't even tell the players first says a great deal about how low his stock had fallen the players he was supposed to lead--as well as his clear disdain for them.
Francona proved that failure with the Phillies wasn't necessarily the end of the line for a first-time big league manager. I guess it's possible that Sandberg could follow Tito's path, working base and bench coach jobs and waiting for another opening, or go back to the minors where he's had success. But I don't see it happening. He describes himself as old-school, clearly comfortable with the sort of baseball lifers stuffed into various corners of the Phillies organization: Dallas Green, Pat Gillick, Bowa. Those types obviously don't see failure as an unforgivable sin, having committed so much of it themselves. But they don't generally hold quitters in high regard. I very much doubt we see Sandberg running a big league team ever again.
And it's worth remembering, given the occasional recurrence of praise for Ruben Amaro's recent GM moves, that Sandberg was Amaro's hire. With Ryno essentially quitting before incoming boss Andy MacPhail could fire him, it seems closer to a sure thing that the GM soon will be moving on as well.