YOU DON'T GET MY POINT AT ALL.
The relationship between these two is ONE ASPECT of the show. Most people don't watch this show for Jack and Kate or any other shipper stuff although some, like me, enjoy that aspect as well as it shows the heart of the character.
Lost is about MUCH LARGER issues that millions of people all over the world tune in every week to watch and there are plenty of other characters that each person prefers. The main focus of the show is how they're going to get off the island, about the island and each episode features flashbacks, which allow us to delve into each character and learn their significance on the island. But things changed in the last episode of Season 3 where the show featured Flashforwards showing the characters OFF the island, what their future is like and what the consequences are for those who got off the island. For example, we know Jack, Kate, Sayid, Hurley, Sun and Jin are the "Oceanic 6", the survivors (according to the public), of the plane crash that happened in the pilot. And then we see one of the characters die and Jack's life become a mess. The farthest flash forward we know of is Jack wanting to go back to the island and this season explains the events leading up to his decision. So Season 5 may feature them going back to the island, yada yada, we'll find out soon.
The show will DEFINITELY continue to be a success because it garners attention from people each day and gains new fans. It will NEVER suck and go off the air before it's time because it's one of the best shows ever been made and is able to cater to anyone with a brain. This isn't like "any other show" and everyone who's watch this show is still watching because we all want to see what happens, solve that which is unclear to us. It's a show you CANNOT stop watching because it literally captures your attention and keeps it until the very fucking end.