Wasn't there nearly an 80 year gap between the NX-01 and the 1701?
We have no idea when the NX-01 will be decommissioned (or destroyed).
I thought I remembered "These Are the Voyages..." implying that the NX-01 was decommissioned and made into a museum ship upon the signing of the Articles of the Federation -- presumably because of the important role
Enterprise had played in bringing about the founding of the Federation.
Besides, if the NX-01 remained in service after the Federation was founded, that presents us with a problem: The canon has consistently portrayed the NCC-1701 as being the first Federation starship
Enterprise. But if the NX-01 continued in service after the Articles were signed, that would mean that the NX-01 became a Federation starship -- and that it would therefore have been the first Federation starship
Enterprise.
However, it is true that there's no particular reason to think that other
NX-class can't have continued serving after the founding of the Federation. The
Daedalus-class ships (which the ENT Relaunch has established is much older than the
NX-class) apparently served into the 2190s. Unless every
NX-class ship gets destroyed in the Earth-Romulan War, they certainly could continue serving well into the 2200s.