Except the NX class would have likely been decommisioned long before the Constitution class was launched I mean we are talking about close a century here.
Not necessarily. There are plenty of examples from the primary universe where ships of an older design were in use for decades after the class was commissioned (Excelsior
Yeah, but they weren't the top dogs any more and. And I doubt Starfleet would technologically stagnate enough that after 100 years the NX class would still be their best ships especially since the already had larger ships even before the Constitution class.