Okay, I really, really like 2016's Rogue One: a Star Wars Story. It's compelling, emotional, action-packed, and a good kind of surprising, putting a spin on one particular thing from the original Star Wars that in retrospect makes perfect sense.
But R2-D2 and C-3PO did not have to be in it.
For years by then (as Revenge of the Sith established), the two droids had been stationed on board the corvette Tantive IV, working for Capt. Antilles, Galactic Senator for Alderaan Bail Organa, and his adoptive daughter the young but already skilled diplomat/ Senator herself/ actual Princess Leia Organa. Soon before Rogue One, per the Star Wars database Wookieepedia (of course), the Tantive IV had been significantly damaged and was in dry dock on board the capital ship Profundity for repairs — understandable, any work in a spacecraft repair yard might draw too much attention from the Empire — where it becomes the best option for getting the copy of the Death Star plans the hell away from the Battle of Scarif. The best of many, many bad options, as the ship's still being repaired and is nowhere near 100% functionality. As Rogue One co-writer Gary Whitta wrote in his short story "Raymus," "For the task of ferrying the most critical Imperial secrets ever captured, they could scarcely have picked a worse vessel at a worse time."* I like that detail.)
So that means the Profundity, with the Tantive IV tucked inside it, is at Yavin, also. (I first wrote "Yavin, too." Maybe it was in orbit around the moon Yavin II.) Isn't C-3PO, by inclination, kind of a homebody? He'd have had to have left the blockade runner, then the Profundity, to be at the base, then he'd have to go back to the Profundity (during an emergency scramble, since HEADED TO BATTLE) and back into the blockade runner, and that seems like a lot of effort for C "No more adventures!" -3PO. Maybe Bail had asked him to join him in the base. Maybe.
And the Profundity didn't actually have to be at Yavin. If you're trying to hide the Rebel base, maybe don't have a lot (or all) of the ships from the fleet there, too! I want to imagine an alternate timeline with it and maybe some other Rebel vessels in a more neutral area, its crew doing plausibly-deniable, no-this-is-not-about-the-Rebellion-again
But no, have the droids at the Rebel base for a callback.
This is a little thing. This is a tiny thing. But it adds stuff that needs to be explained, like how late in the original Star Wars (A New Hope, for those scoring at home) Luke and R2 are at the Rebel base preparing to board an X-Wing to attack the Death Star, and the technician doesn't recognize R2: he mentions that the droid's rather beat up and suggests replacing it. Yes, R2's in less-than-great shape after his adventures and misadventures on Tatooine, the Millennium Falcon, and the Death Star, but he's not unrecognizable. Okay, maybe there were so many people at the base that this tech never saw him before then, so I guess you can explain it. I guess.
(I'm realizing how tiny a thing this is as I write. Interactive blogging!)
There's a lesson here that, from what I've heard, the makers of Rogue One learned and not everyone who makes prequels has learned (hi, makers of Fantastic Beasts): it's easy to add too much to your prequels. Tie them in too-detailed ways into the parts of the stories you've already told ("this explains that!"), and you risk adding details that will make people "wait, if that happened, wouldn't that mean that other thing couldn't have happened the way we already saw it happen?" It's delicate work! It's kind of like the pitfalls in telling time-travel stories. Does [x] keep [z] from happening?
I can't be the only person who's wondered this. So that means someone's explained this all much better than I have. No biggie, I still enjoyed writing the entry.
* From the book From a Certain Point of View: 40 stories celebrating 40 years of Star Wars. Fun book!