But also in LOST's case, they were actually making it up as they went a long at the start. Some time around season 3, there was a huge announcement that the show would end in 2010 after six seasons. It was then the showrunners had to somehow figure out how to bring all of this together over the next three seasons. This was when the series began to really collapse under its own weight, I thought.
My recollection of LOST is that it was more coherent in the early episodes, advancing the character arcs nicely along their journeys of redemption, but then when the show became a hit, ABC asked them to slow their roll and stretch out the storyline longer, so the characters' growth suddenly stalled or regressed and it was just treading water for a while. That was when I started to lose interest. I think I gave up during season 2, or early 3.