Re: At what point does the Federation decide to stop building Enterpri
After the Naval Construction Contract is up, the registry can be anything, and is merely used for display purposes on the hull and identification purposes in status reports and whatnot.
IF NCC stands for "Naval Construction Contract" - which is totally fanon but has always been my preference as well - then you've actually pointed up a reason
for the use of the added letters: each new (or even just renamed) ship named Enterprise becomes a lettered addendum to an existing contract!
If that were true (and at this point, we're going down a rabbit hole of "what if", but I'm game if you are, and that's kind of what this WHOLE conversation is, anyway

) then perhaps the hull code would change if a different "company" (not sure exactly what this word means in the 23rd century and beyond, but hope to find out) built a ship called Enterprise and thus started a new contract. (Maybe the "company" is a shipyard group in the Sol system - so a different "company" would add a new contract if an Enterprise were built at, say, Andor.)
But regardless of that, I think a new hull code might be called for if the Federation and Starfleet ever became part of a new polity - such as what might happen if the Khitomer Accord worlds and the Typhon Pact governments ever joined together to unite the Alpha Quadrant. In which case, I don't see the pattern lasting very long after the -J, if to that. (As others have pointed out, the -J might be from an *alternate* future.)
And I will self-indulgently point out that in my own fanfic Trek future, I decided that Starfleet reserved NCC-170100 for the latest Enterprise when the timing was right, so that would tidy up the issue nicely, too.
