So you assume their plan the whole time was to disappoint us or do something bad?
I take it as, what we leave unanswered wasn't the main focus to begin with but rather how the characters end the story is or we wouldn't have spent more time on the flashbacks/forwards/sideways more than the mysteries itself. Just like Trek spends more time on the cast than the idea of space.
I'm so tired of people repeating the tired refrain that it was all about the characters. The Mythology was as important, if not more so than the characters. There were a lot of times where the story was pure plot and the characters were simply being moved about like chess pieces or reacting. Deaths were plot points not given the big emotional fanfare--the only real exception being Charlie's which worked so well in part because it was given the focus it should have--counter this to John Locke's.
S4/5 were heavy on exposition or giving us a timeline or exploring the history of the island. Yo go back and try to do review of those years and you'll find yourself mainly writing up a synopsis of what happened.
So yes for me all of a sudden so abruptly changing gears back to characters and halting so radically the momentum they had got going back in season 3 when they knew the end date and discovered a newfound sense of narrative purpose and carried through to the final moment of "The Incident" in season 5--did in fact feel out of left field and not consistent with what came before.
Yes I know in your traditional series that the characters are what is important and usually when a series ends that the final season is all about nostalgia and sentimentality but for LOST I think it was far more important to wrap up the plot arcs and not focus so heavily on characters--many of which had not been the focus for years or who ultimately had pretty poor character arcs this season ie Kate, Sayid or Claire for example.
A lot of the cameos didn't work the way they did on TNG's AGT because a lot of the LOST characters they brought back were nothing more than plot devices to begin with.
At the very least they could have done a better job balancing the plot/mythology with the character stuff if they were determined to make character such a centerpiece this year.