Oh, I don't doubt that Jane was horrible for Jesse or that he wouldn't have survived long if they were able to stay together with all of that cash. I'm even willing to entertain the argument that Walt's decision, cold as it was, was ultimately the right decision for Walt and Jesse (it obviously was the right decision in an absolute sense, as evidenced by the chain of events that ends the season). However, I don't buy that Jesse's well being was the primary motivation for Walt's inaction in that moment. I agree that he sees Jesse as a son and that Walt genuinely cares for him, even if only on a subconscious level, but a drugged out Jesse and his junkie girlfriend are ultimately a problem for Walt and I think that Walt's decision is based on solving that problem and on protecting himself. That it removed a negative influence from Jesse's life and allowed Walt to get him into rehab was, I think, a secondary benefit.
Also, no matter the reasoning behind it, I just can't help thinking that if someone can just stand by and watch that, there really isn't much left inside that's human.