Jabba’s appearance was unnecessary like you said. it didn’t affect the plot in any way. It literally could have been any gangster in his place just like the droids, and the Fetts. They make the galaxy feel smaller because it seems like there must not be a lot of gangsters, bounty hunters, droids, etc in this entire galaxy originally set 50 years before the OT. Nothing evolves or changes in this supposedly living world. IMO Boba Fett’s mystique was lessened by his inclusion, and they could have gotten anyone to be the base copy for the clones. Have they really been using the same model of astromech and protocol droids for 100 years? Chewbacca is as unnecessary as Jabba was. It could have been any Wookiee who helped Yoda. Owen and Beru did what they needed to in the OT, but again, they have been on the same farm for 50 years? How boring. There was no rule that said they actually had to be related to Anakin or Luke in any way. They could have just been someone Obi Wan trusted. They could have even been part of the crew of the spice freighter Luke was told his father served on during the war. There could have done a whole adventure in the PT where Obi Wan and Anakin were being smuggled on a spice freighter piloted by Beru and Owen at some point which would have even given the lie a kernel of truth. Anything would have been more interesting.