From my personal experience inferior Si is more about being deeply worried about not having anything on the horizon to fight for, no new projects and experiences, like stepping into the swamp and getting sucked by quicksands, if Si doms are about recollecting facts to resist the thunderstorm of the future, Ne doms need to be sucked in by it and destroy the bunker Si dom have built. If future doesn't happen our passion would be reduced to a concrete metal structure, caged spirit, passiveness, our Si coffin, the boredom that present is.
Imagine you are going to travel, you missed the airplane because your mind was already there, in a passive way not awake yet, like chutulu, waiting sleeping, your body was a machine finishing the procedures required to complete the transition to the new stage. But once it turned up, it realized it had failed in an important task (Inferior Si)
So auxiliary Fi activates tertiary Te and this starts making economic charts, calculating costs, hours, human beings into trampolines, material conveyors, and then... is it worth all the effort? the ultimate question before Ne reactivates. Damn yes.
But as soon as future begins to happen you are already worrying about the next challenge.
In conclusion, Si is negative about future, Ne is possitive.