You may be right. But given that this game is most likely a linear corridor shooter, there will be no incentive to do a replay without multiplayer, unless the devs implement RPG mechanics or moral choices akin to bioshock that affect the outcome of the story.
I suppose we'll have to wait and see until decent gameplay footage is released.
I saw an RPG mechanic in one of the videos and it relates to your playstyle. The more you use the knife, you gain the ability to throw knives, the more knives you throw, the more you can carry, etc.
The only skill tree I saw was stealth, I can't recall what the others were. We'll see more about it later I'm sure.