Carlton Cuse, one of the two showrunners for Lost was the showrunner for a show many years ago called The Adventures of Brisco County, Jr.. It was an oater with some SF elements, kind of like Wild Wild West. There was an overarching storyline about a gold orb and a villain obsessed with it. The show was cancelled before it was ever explained well.

In recent years, Cuse gave an interview in which he mentioned Brisco and the gold orb plot. He said that the writers had no idea where they were going with that storyline, that it caused a lot of problems, and that he was never going to do that again. (I wish I could remember where I saw that interview. It may have been in the special features of the Brisco DVDs.)

Now, he could have been blowing smoke up our asses, but the plot elements of Lost hold together pretty well so far, with a lot of callbacks to seemingly unimportant details from distant prior episodes, so I'm inclined to say that he does know where the plot is going. In addition, they have a contract for about 32 more episodes over two more years and the show will end (and they knew that 16 episodes and a year ago) so if they didn't have an endgame before, they have had more than enough time to come up with one.

I know that your argument wasn't that it didn't have an ending, but that it was not in sight, so sorry if I'm arguing a point that you weren't trying to make. Still, I think a lot of people have learned something from The X-Files: that it's a bad idea to go into a show with a story arc and not at least know some major plot points and the ending.