"All of this is why I think that Jacob is the bad guy in all this."From their exchange in S5 finale, we know that Jacob wants the best for the Losties while MIB has only selfish desires. Jacob thinks humans can live without wars, destruction and corruption. MIB sees only our dark side.
If one is playing God and the other the Devil, then Jacob is "god". The question becomes: Is God good? Well, we know Jacob has ulterior motives. That's why I say most viewers may not like the outcome!
"Look at their lives after Jacob's influence, and compare it to their lives after everything is undone."
The flashbacks showed us that all the Losties were miserable but they had dreams. I see the flash-sideways, the world without Jacob, as a world without god. Look at what you've just described: All our Losties are content, they have mostly what they want but are they happy? They seem to have abandoned their dreams and are just going through the motions.