I think some are just bitter that the other series lasted longer
From the first airing of The Man Trap in September of 1966, through the last movie appearance of Kirk and two others of the main TOS cast in Generations in November of 1994, it was 28 years.
On the other hand, between Encounter At Farpoint (September of 1987) and the end of the TNG era with Nemesis (December of 2002), it was 15 years.
TOS might not be the only "true" Star Trek, but it is undeniably the heart and core of the Star Trek world. When Spock appeared on TNG it was a event, when Riker appeared on VOY it was a blip. When DS9's crew time travel into one of a previous series episodes, instead of the Tribbles episode, they could have been inserted into a TNG adventure, but they weren't.
When the ENT mirror universe got their hands on a future starship it was a Connie, and not a Galaxy/Intrepid/Defiant.