Thinking Luke’s return would be a big deal makes sense in-universe. I mean, of course it does. Luke is the last Jedi. It’s a pretty normal reaction for the characters to have to Luke being gone. Luke not really wanting to return is also the most logical conclusion of what was set up in TFA: “Do you think that I came to the most unfindable place in the galaxy for no reason at all?”
This weird internet mythology that Rian Johnson’s prime motivation in making TLJ was to completely subvert expectations is just not true. He just said that phrase once behind the scenes and it got plastered all over the marketing. Rian constantly spoke about the exact opposite: TLJ was made as a straight line continuation from TFA.