I mean, he’s everything, always (same as C3PO), so what’s stopping him from making another appearance just for the hell of it? I was just watching Rebels again and forgot that, true to form, the dynamic droid duo shows up incredibly early in the series, just like they do everywhere else.
They are also the only characters who have appeared in every single Skywalker saga movie, so it’s safe to say that they are established as it gets. Plus, it’s not like it would be that hard to figure out who should play them. R2-D2 doesn’t require anyone, so if nothing else, he can just make an appearance, and it can be assumed that somewhere nearby is Luke Skywalker, since they never go far without each other (until the sequel trilogy).
That way you get a bonus two-for-one. If R2 is there, surely Luke is nearby, even if we never see him. Maybe we can just see his X-Wing or something clever like that.
It’s also an appearance with the least risk. Put in R2-D2, who has no speech and only has personality from his beeping and antics, and people will smile and continue on with the story. If you start putting in other characters like Lando or Dengar or even C3PO, you have to give them a bigger role, and that runs the risk of being too much of a fan service. If that’s ever really a risk in the Star Wars universe.