If you look at the OT you have Vader, Ben, Beru, Owen, Threepio, Tarkin, Artoo, Yoda Mon Mothma and Palpatine definitely from the previous generation.

Even if the PT hadn't happen you could imagine them in an unmade history of the same universe.

Of those Owen, Beru, Tarkin, Ben (sort of) were taken out in the first film.

Yoda is 're'introduced in the second film and taken out in the third.

Mon Mothma plays like a cameo from an important character from the past and Palpatine (like Jabba) goes from being an unseen reference to a growing presence which is fully revealed in the final film.

I can imagine the OT characters coming in and out of ST in a similar fashion with only the droids being in every single film.

In this respect I'm not that put out if some of the OT cast (Lando, Wedge) aren't in Episode VII as they could end up doing a Mon Mothma and cameoing in Episode IX.