Unlike many fans, I believe the sideways timeline isn't the true timeline. It feels warped to me, and, therefore, is unsustainable.
Let's go back to the record analogy used by Faraday to explain the flow of time after Ben jogged the donkey wheel off its axis. Faraday compared time skipping to a record skipping.
The record analogy can also be used for the sideways universe. Except, rather than having a record that's skipping, going back and forth over a few grooves, we have a warped record--one that's been reshaped a little.
Maybe whatever happened at The Incident caused a bend in time. A warp in the record, a distortion.
I think the sideways universe is "bent" because things seem "off" to me. Just weird little stuff. Such as:
--Helen never impressed me as a woman who'd paint her nails black. What's up with those nails?
--Hurley is smooth. I don't recognize the Hurley talking to John in the parking lot. And he drives a big, yellow Hummer?
--The woman who was going to adopt Claire's baby reminds me of someone from a bad dream. It's as though she's not really there. Very hazy, pasty, ghostlike.
--What's with Desmond coming and going on the plane?
--A hazy mirror is shown next to Kate when she opens Claire's suitcase.
--Ben and Locke meeting in a high school faculty lounge. Ha!
I'm sorry I don't have more concrete, strong evidence to support my theory. But, to me, the sideways timeline doesn't feel like the Losties' destiny. It feels wrong. Not meant to be.