We should not get stuck with the idea that success would be the opposite of failure here, though!
In the reality of, say, WWII, several major ship types were considered failures, having been designed or built all wrong for their intended task. Perhaps such a task had grown completely outdated, perhaps the initial specs were wrong, perhaps the workmanship or resources allocation was poor. Some of these ships served the winners, some the losers. Yet the winners did not win thanks to their less failed ships, nor did the losers lose because of their failed ships. It's just that in practice, the many shortcomings of the ships of the losers are enumerated afterwards, while those of the winners are glossed over.
It just wouldn't do to stop operating a failed ship, is the lesson to be learned here. Having a poor ship is better than having no ship at all, in many situations. And apparently in all of those concerning Starfleet - after all, the fleet scenes feature their share of antiquated types, with nary a comment. Apparently, those don't drag down the fleet, but add to its numbers, which are the only thing that matters. Much as in WWI and before...
Perhaps the Galaxy fell 78% short of the hopes Starfleet set on her, and nevertheless added greatly to the Fleet. Perhaps the Galaxy was 98% the technological success she had been intended to be, but had no purpose in the Fleet after practical experience had shown the powerful ship weakened the overall war effort by depriving other, more needing parties of resources or skewing tactics or concentrating firepower when distribution was needed. Such things would be independent rather than interdependent in the end, and the history of the Galaxy class could be equally well be written with the words Success or Failure in the title.
Timo Saloniemi