You kind of automatically put Venus and Serena ahead of the Belgians again, and if Venus plays a full (to her standards) schedule that might well happen.
People don't seem to notice that on average, and when winning those, Serena picks up between 15-20% of her points at the GS.
Failing to play one (the US Open in this case) has her losing about 17% of her points, and see her fall over 1000 behind Clijsters.
By the time she comes back she'll be probably about 1500 points behind Clijsters.
Of course we cannot make any predictions on Clijsters, if she suddenly gets injured things might change quickly, but if she manages to do not much worse next year than she did this year, it'll be hard for Serena to catch up. If she doesn't play 1 slam, she has to make up for it at other events. Considering she hardly ever loses, she can't really do much better at many tournaments, so if she wants to get the no. of points she's having today (which might be needed to overtake Kim) she'll have to add several events she usually doesn't play, and I personally don't see that happening.
The way Serena earns her points, she needs to compete at all the majors, and doing well at them.