that could be a result of Starfleet wanting to honor the loss of that ship and its crew for awhile before launching her successor
Or then Starfleet thought that the name Enterprise
was jinxed, as the E-nil, E-A and E-C had all supposedly gone before their time, and the E-B may very well have suffered a similarly embarrassing or horrifying fate...
Starfleet knowing back in the 2340s that the Galaxy
class was coming? Doesn't match the TNG Tech Manual ideas or anything in canon. And by such a token, there should never have been an Excelsior
class E-B, as the name should have been kept waiting for the "upcoming" Ambassador
Does the creation of the Federation and Starfleet necessarily mean those individual miltaries go away?
The dialogue concerning the fate of the Bajoran militia would indeed seem to spell out that this necessarily happens.
Although the old Founding Members might be exceptions to this later rule, of course. But outside FASA, there's no suggestion that Andor would maintain a separate Blue Fleet, and any evidence of a separate Vulcan force is ambiguous at best.
For example, in TMP, you have Spock traveling in a Vulcan designed shuttle.
...One with phasers identical to those of Kirk's ship, and a docking port compatible with those on Kirk's ship. So the design sort of indicates servitude rather than equal partnership... Although we can speculate who is subservient to whom. Vulcans might be the leading shipwrights of the Federation, having introduced the saucer-and-nacelles design by helping build NX-01 and perhaps also her predecessors; humans could still be the leading soldiers. Or then Vulcan shipbuilding has all but died out by the time of ST:TMP, but these "intellectual puppets of the Federation" are in fact the top politicians controlling the joint military.
Much remains to be learned of the setup, but statistically the onscreen evidence keeps mounting to suggest that humans dominate every aspect of Starfleet, that other UFP cultures contribute very little that would be visible, and that there are very few "odd jobs" that would exist outside the joint Starfleet's jurisdiction.