I've always been in favour of the ER, CSI, professionals doing a job and that was their place of work for a while approach. They get transferred, or leave the service in variety of ways.
TNG could've been that, had either Picard stayed assimilated or Riker accepted either of the Captaincies that came his way. In a sense, there was promise early on when they took the step of killing off Tasha Yar and allowed a change of ship's Doctor the following year.
If the series lasts, they can always ask the original lead actor back for the final episode. Either back as their character, or as an alien... under make-up, to honour the part they played in making the show a success in the first place.
I also think the principle cast could be streamlined down to four or five, with others that come and go. Since those Travis Mayweather & Harry Kim guys don't get a fair share of the action across their runs anyway. Better not to sign them up for seven years and have that option of releasing the actor, if the character is neglected.
Even those moderately interesting, through their friendship with another character, like say a Malcolm Reed or a Pavel Chekov could do with having the option of going out in a blaze of glory.