Um, wasn't part of the foundation of the modern Trek litverse layed down by Star Trek: New Frontier and its mostly lit original cast. We also had the Stargazer series (with only Picard who was very different from the TNG version), COE, IKS Gorkon/Klingon Empire and Vanguard, in fact the DS9 relaunch cast is mostly made up of new characters. Seems like there might by a problem with your theory there.
Not necessarily: Those series you named started out as Trek Lit-only. People who pick them up know what they get. The novels based on TV shows, that's a different story: Long-time fans expect to see the characters they are familiar with (cf. success of TOS set during the everlasting five year mission).
Though, you are correct with the DS9-R. It had/has a nice mix of old an new characters.
Most of the TV characters can be active officers for the next few decades. Picard was already a dinosaur when TNG started and 24th/25th century medical science surely can make people live centuries. In addition, characters could still be part of their series when they leave the ship (e.g. future Ambassador Picard).
/my 2 cents