Lets be honest, who from the original trilogy are they going to have? Ford hated playing Solo and wouldn't come back if they put a gun to his head. Fisher had that whole blowout with Disney back in 2016 (please don't start that debate back up again here, I'm just using it to demonstrate a point), so she's obviously not going to be interested.
Hamill demonstrably was willing to work with Disney. But lets be honest: yes, casting him as Tarkin was a stroke of genius, I know that was a fan-favorite performance and I'm not dissing it, obviously he's still on his game. But do we really think he's still fit enough to pull off Luke again? Christopher Lee was straining himself to play Dooku, and Dooku wasn't the bloody hero. And frankly, Mark Hamill is no Christopher Lee. Sure, Luke would probably be in a more Obi-wan style role anyway, but we already saw in their remake that Disney's not willing to let actual Obi-wan sit around and rest his bones, so forget Skywalker himself.
If there were gonna be sequels, the original character's fates would probably make for uneasy viewing.. I know Disney doesn't really "do dark", but it's hard to avoid given the casting dominoes that start with Harrison Ford.
If Harrison flat-out said "no", Han would need to be killed off in the title-crawl for episode 7 (or killed off on-screen pretty promptly if he appears).. heck, best case scenario he makes an inconsequential cameo right, probably at the end. I suppose he could have just deserted the rebellion, but that's as bad as being dead for the other main leads.
So Han is absent or killed-off.. well then Leia is probably either distraught & disillusioned (a shell of her old self? ), or nihilistic and reckless.. either way, it's glum.
As for Luke, maybe he's still optimistic.. training a new Jedi Order, trying to get things back to how it was in the old republic - but in the messy aftermath/power vacuum of defeating the Emperor, there's gonna be a lot of chaos, a lot of little fires to dampen down - wouldn't that wear him down over time?