One problem I see for Sanders is that any loses before California make California's importance less valuable.

Between now and California there are about 870 pledged delegates available. Now Clinton needs only about 630 delegates (super + pledged) to secure the nomination. If she just wins half the delegates (which is not exactly how it works, but it gives a rough guess) that's about 435 which brings her under 195 delegates needed. On the day California votes, there are about 700 delegates available and even if she only wins 30% of them, she will still cross or get so close to the threshold. Ultimately proportionality hurts Sanders because he is far behind and Clinton still does well. He needs wins and needs to win big, while Clinton can afford to just split 50:50. And any victories she has makes Sanders' chance of victory more and more difficult, and that is from an already difficult position. I honestly do not believe Sanders can pull off large victories he needs in places like NY and Pennsylvania.

You can argue that superdelegates can change their mind and the like but this is where it stands now and superdelegates do count. Plus as she gets closer and closer to the threshold, she has a very strong argument that she will win and why would her superdelegates change to Sanders when they are supporting someone who is winning?

To me, NY is Sander's last chance. Anything short of a victory will effectively end his shot. People will still hope for California, but honestly I believe it will be false hope. The scenarios just become more and more absurd and a lot happens in between NY and California. It'll be interesting to see regardless.