Actually, I think it makes sense. The Constitution is the culmination of Project starship, designing the workhorse military cruiser / explorer / all-around best ship possible with bleeding edge tech, and modular design.
So the Hermes and Saladin use Constitution saucers and nacelles as spare parts that get used as smaller starships until needed to repair a Connie.
That would be to assume that no other starships OTHER than the Constitution class existed before the Constitution was put into service. That is not a logical assumption. That also assumes some sort of standardized mass production of the Constitution class' many components to achieve an economy of scale; that is ALSO not a logical assumption, considering they only built thirteen of them.
The predecessors of the Constitution class, for that matter, would have been built with completely different components and designs, so there's no call for the assumption that designs "diversified" at some point in the future. They would have been there from the beginning (and obviously WERE, judging by Enterprise) so the standardization of Constitution-style kitbashes wouldn't have been all that useful.
You might have a lot of ships with similar shapes and styles as the Constitution class but with smaller saucers/nacelles/engineering hulls. So if we ever saw the canon (as opposed to fanon) Saladin class -- assuming the Kelvin isn't already the exemplar for this, and that is a BIG assumption -- it would probably look like simply a smaller, more compact version of the Enterprise: a saucer, two nacelles and a secondary hull, but being only 150 meters long and having only 14 decks.
If they hadn't seen that kitbash ships weren't as useful as purpose-designed ships previously, due to an earlier administration's short-sightedness, they might have only approved the 6 commissioned.
They obviously DID, since we saw far more than that in service in DS9.
Also, the "kitbash" component of the TNG era isn't pronounced enough to make your point; there are DOZENS of starship designs in that series run, many larger or smaller and with different mission roles. You could say the Nebula class is a derivative of the Galaxy, but it doesn't use precisely the same components.
More to the point: the whole reason for ships like Saladin and Hermes is because trek fanon has a lot more idea-guys than it has decent artists. We're very good, as a group, at coming up with ideas for what MIGHT exist, but we're very bad at coming up with compelling visuals for what that something might look like. As with canon itself, the ideas and the visuals don't usually match all that closely.