OTOH, the two ships could be meant for the very same mission and situation. It's just that they take slightly different technological approaches - one covers with quantity what it cannot afford to do with quality.
But it could also be argued that they were always intended to fight in completely different leagues. The E-E certainly looks more like a successor to the Excelsior
class than to the Galaxy
one - possibly inheriting the former's rather militant TNG era role. Which makes one wonder what sort of a successor Starfleet has got planned for the Galaxy
class, and when it's scheduled to come out. The early 2400s, perhaps?