It's doubtful the Kirby estate could find a better attorney, Toberoff is probably the most successful IP lawyer around.
A different court might yield a different result. As we know even the US Supreme Court is often sharply divided, the outcome of many cases hinges on the judge or panel of judges.
Here is a bit of analysis from the LA Times which points out the judge based her ruling on Lee's testimony that he created all the core characters and plots, and then passed them on to the artists.

LATimes:
"But Marvel argued in legal documents that his illustrations constituted "work for hire" — that the comic book covers and artwork he drew were created under the direct supervision of Marvel and its famous editor, Stan Lee. It was Lee who generated the plot or synopsis from which an artist created pencil drawings, according to court documents."

In other words Lee's testimony in this case was Marvel's Superman.