Well, I don’t think that explanation was never actually mentioned in the show (not that it really makes much sense anyway.) But what was mentioned was that all current Federation starships are networked together, implying that older vessel classes from the Akira back through to the TOS/TMP era are no longer in service (if the ‘relics’ Shaw refers to in the museum are any indication), and that those decommissioned ships could be used as raw material for new ships. Of course with replicator technology this would logically not matter.