Yeah, it's a little like the Doctor Who regeneration retcon: when regeneration was first introduced, the Doctor told us "Time Lords live forever barring accidents", but later (it was claimed) 12 regenerations was the limit. The obvious retroactive continuity was that Time Lords can live forever, in theory, but if they have 13 accidents then they won't.
It's a little similar with the Galaxy class. They're supposed to be able to sustain themselves for 100+ years, but due to the nature of Starfleet and the sorts of situations they frequently find themselves in, chances would be pretty rare of a ship ever actually surviving that long. Enterprise did well to make it to eight, Odyssey only made it to seven, and Yamoto was dead in the water within the first 24 months of its life.
It's a little similar with the Galaxy class. They're supposed to be able to sustain themselves for 100+ years, but due to the nature of Starfleet and the sorts of situations they frequently find themselves in, chances would be pretty rare of a ship ever actually surviving that long. Enterprise did well to make it to eight, Odyssey only made it to seven, and Yamoto was dead in the water within the first 24 months of its life.
