On the other hand, cruisers have never been particularly large as ships go. They just happen to be built all wrong to serve as freighters of that size, say. In the Trek future, there might be civilian ways to make use of cruiser hulls, ways that have no counterpart today. A bit like a cruiser of the 19th century could have been utilized as a merchantman, to stretch another Trek analogy.
Timo Saloniemi
This is a great explanation as it explains the fact that the desktop Constitution-class Models in TNG have the window insert parts removed and replaced with cargo doors. The sideways nacelles give another clue.
Perhaps it works like this:
Let's call the TOS architecture type 0, the refit architecture type 1, etc.
Maybe a TOS ship can be updated from version 0 to type 1 or 2, but not on to type 3,4, etc.
Perhaps a newly-built ship of refit version 1 can be updated to are 2,3,4 and beyond.
Thus, Constitutions could not be refitted again and again, but Mirandas and Constellations could. By DS9, we a type 3 or 4 Miranda, because those were new builds in the 2290's, but ships from the 2240's-2260's could not be updated again. That would mean Centaurs were never made in high numbers because they only needed to replace Mirandas that could not be refit again. The various revolutions of Bridge Modules would support this, especially since the new Bridges with ready rooms and fewer consoles had to fit in a dome that looks the same from the outside as an older bridge.
Meanwhile, new tech was TESTED in Constitution class hulls, but they would not have handled it for acutal service, so they later became cargo ships. Heck, this sort of approach explains why the Wanbundu class is called both a freighter and a cruiser in TNG.
The point of the "Phase One" terminology is that Finney wasn't found with the tech, and then they characters have to deal with it.
