The show's creators always said that LOST was planned from beginning to end - but in a way that was flexible enough to adapt to new ideas and circumstances along the way without sacrificing the Big Plan.
The thing is, for me anyway, I just I don't believe them. I'm kind of an asshole in that if you lie to me once, I'm very skeptical of everything you say from there on out. In their case, the "Oh this is not a sci-fi show, everything can be explained through normal science" bullshit line has made me VERY distrustful of anything they say.
They may very well be telling the truth about having it all planned out, but I find it hard to trust them.
The problem as I see it is that IMO the plan was to initially make us think LOST was a standard - if a bit strange - drama, and over time move into the sci-fi arena. So to the creators, it always was
a sci-fi show. But one that kept it's powder dry on that front for a while.
But the fans very quickly ('cos we're clever like that!) sussed out that LOST was indeed a sci-fi show - so what could the creators do when confronted? If they admitted it, then we'd have said "Alrighty then! Give us the big sci and indeed fi! And do it now!"
. But LOST is a slow-burn concept that needed to take it's time to get round to the juicy sci-fi. So they said "Nope! No sci-fi here!"
. They had
That's my take on it, anyway.