The 12th Fleet: Starbase Valhalla

My theory is...
At the time the Enterprise-J was built, the expanse was said to have grown 50,000 light years in all directions and still growing, that expanse would have displaced a lot of Federation planets and their population, those "refugees" would have needed a place to go and perhaps Starfleet might have built ships like the "Big-E" to house some of those refugees and possibly explore far into deep space to seek out other planets to colonize.

Yet because Archer succeeded in stopping the Xindi and the Sphere builders, there wouldnt have been an expanse hence no need for the J to be built.

The thing is how many people can they fit in to a 2 mile squared area??? Ok yeah the J has multiple decks but that's still nowhere near the population of a single planet. Also a lot of the space on the J will be taken up by systems, crew quarters (i'm assuming it has a huge crew compliment), a massive warp core etc.

I think the need for the J disappeared when the Sphere Builders were defeated. Starfleet never required a massive ship so they continued to build the normal, less intimidating ships such as the Sovereign.

Can you imagine Starfleet sending the J on a diplomatic mission to a potential new ally's homeworld??? It would scare the bejesus out of them as soon as it dropped out of... well whatever drive system it has.