Nothing substantiates that in regards to SF ships of the same class that leave construction yards.
AND NOTHING SUBSTANTIATES THAT
OPINION ANY MORE THAN MINE.
And a *sigh* back atcha pal. I don't know why you're taking this so seriously, we're just postulating fiction that has no canon referent whatsoever. Neither of us is correct OR wrong, both our opinions are equally as likely. I'm just throwing out a "what if" thought process here. Although you seem to be working from assumptions that the real world doesn't support.
The five space shuttle orbiters were each substantially different from the other. While some parts were interchangeable, a LOT were not. No SR-71's instrument panel was identical to another's. No Nimitz Class carrier is 100% the same as the previous build. Etc, etc...
Some of the hypothetical variations on the GC I mention that you casually dismiss with refitting would require such extreme rework that it would be pointless. Say the main EPS interconnect trunk on ship 4 is moved 50 meters to port simply because some engineer realized a handier way to route it to make final assembly at the shipyard easier. This in no way invalidates the current position of the trunk on ships 1 thru 3, because they're already assembled, and rerouting the trunk would be expensive and pointless, not unlike this thread. All the ships work fine, but that leaves 4 thru 6's saucers unable to interchange with 1 thru 3's - an operation that Starfleet considers so unlikely anyway, as to eliminate any point in modifying the older ships.
I await your next rude attempt to treat me like an ignorant child.