The bittersweet ending of ROTJ would have been celebratory, but with the wiseness of life experience. you win the battle, but you remember that it wasn't free. LOTR sort of ends on a similar bittersweet note, and its very poignant.

But a big part to remember is that the ending described there probably fed into a sequel trilogy. So its not the end-end. You want to have some elation to cap off the trilogy, but things have to be complicated still, because there's a whole 'nother trilogy to actually complete everything. So, Luke walks off like Clint--great! We'll see him as an old man in the next film. Wouldn't that have been great, to have Luke walk into the night now as a Jedi, having earned his maturity, and then we see how that moulded him twenty years later as he tries to figure out how to continue the Jedi way for a new generation, now with some grey in his hair.

I think that sort of ending makes more sense in the context that its the end of a trilogy but not the end of the storyline and not the end of the characters.