For me it isn’t just fan service. We saw grogu evolving as a jedi, and he being present in the attack of the jedi temple is a new detail that we don’t knew until now. Seeing Luke as a true Jedi master, but also learning how to become a teacher and building his jedi academy, seems like character development to me (and a bit of an apology for his treatment in the sequels). I think a big part of the fandom always had wish to see this part of Luke’s history, that we just had read in novels or comics, and I’m really glad that we receive this in the live action format. I totally agree that doing that in BOBF is wrong though, but for me at least, the episode did moved forward the story of these characters (and for Luke, is a kind of a utopian dream coming true due to his age, thanks to the AI technology).

Cad Bane and Cobb Vanth for me aren’t fan service, as they’re one of the few things that totally fits in the narrative of BOBF since episode 5. Ahsoka was a bit gratuitous though, she really was stuffed in there.