A lot of things having to do with the poor writing of this television series irritated me over the years, including the ending which cheated an invested audiences of their answers.
However, think about season 6 without the FST at all. We would just have the Losties on the Island, and in the end, some of them would eventually escape.
What's wrong with just ending it there, with the plane flying away?
The whole land of the undead all season long didn't even need to happen.
In fact, instead of zombie land for the past few months, they could have spent that same time answering questions and ended the show appropriately.