The first thing I have to say is that the writers left the ending open to interpretation. We fans will forever be debating on the entire show's meaning.
What I gathered from the finale was that some people died before Jack, like Boone. Others died after Jack died, like Kate, Sawyer, Hurley and Ben. Some obviously lived long after: Kate said she had waited a long time to see Jack; Hurley and Ben with the whole #1 and #2 conversation.
Since there is no sense of time in the FS (I don't know what else to call it), even though people died at different times their souls are all there at the same time.
As Christian said that the time they spent together were the most important time of their lives, they all needed to be together in order to finally move on.
The people who weren't in the church, like Miles and Alex, weren't ready to move on. Ben chose to stay behind because he felt he didn't deserve it OR he wanted to try and help everyone who wasn't in the church, like Miles and Alex, remember their time on the island OR he didn't want to move on, he wanted to stay with Alex and Danielle.
Eloise didn't want Desmond to take Daniel because she was happy with her afterlife.
The FS wasn't purgatory, Heaven, Hell, or any other form of afterlife. My theory is that it is actually sort of an in between. Since they all needed each other when they were alive, they needed each other to move on; Sawyer needed Juliet, Claire needed Charlie, Rose needed Bernard, Desmond needed Penny, etc. etc., and they could have moved on without everyone else. Shannon wouldn't have needed Juliet because they never met. However, everybody needed Jack in order to move on, and Jack needed them. That's why they were all waiting for him. Either that or they wanted to wait for him because they were friends. I prefer the former.
In the season 6 premiere, LA X, in the beginning where Jack and Rose are talking up until the plane hits turbulence is actually the FB, and after the plane makes it through is where everybody's "afterlives" began. In other words; first came the flashbacks up until turbulence. Then came the turbulence; which, figuratively, everything starting from day 1 on the island until the last main character's death (whoever that is, could be Kate, Hurley, or anyone else who made it past the end of the series) happened. THEN everyone began their afterlife with them on the plane after it made it through the turbulence. I'm sorry if I confused you, I confuse even myself whenever I try to explain this.
In regards to Jughead, it didn't cause the FS (duh).
And there you have it, my final theories. I was satisfied with the ending, although I wasn't at first.
P.S. If you noticed, a lot of the final farewells fans submitted that were showed on TV had to do with "moving on" or something similar.
P.S.S. If all you’re going to do is bitch and complain because you didn't like the ending, don't even bother commenting.