Statutory rape and rape are not the same thing, the penalty for the former is much lighter. I think the charge in California (if I'm reading the penal code correctly) would be something like "Unlawful sexual intercourse with a minor". Which is a misdemeanor if the age difference is less than three years, and a felony if more. It is punishable by a maximum of one year in prison if the minor is under 18, or up to four years if the minor is under 16. There are also civil penalties (ie fines) that could be levied, a max of 25k, depending on the ages of those involved.
Without testimony they would only be able to prove one charge (in the case of Angel's conception) based on 261.5(d), and perhaps one charge (in the case of Starlet's conception) based on 261.5(c) based on DNA, for a total of 5 years at most. And in Starlet's case, since she was born around when Jaycee was 18, and since proving her birthdate is entirely dependent on that missing testimony, they would have a hard time proving that charge at all. Plus, this would only apply to PG, not Nancy. This is the worst case scenario, but it could happen.
It becomes rape when done under duress (kidnapping or confinement would qualify), by force, or the victim is incapable of consenting due to physical or developmental reasons. There are a host of other things which qualify as well. These are the charges made against PG and Nancy. If the state can prove kidnapping and/or confinement, it would not matter if she co-operated at some point, it would still be rape. But, if for some reason testimony does not materialize at trial, they would not be able to prove that, and would have to fall back on the unlawful sexual intercourse with a minor charges.
This is why the testimony is so important, it could have a huge impact on the case. In my opinion it would be irresponsible for the prosecution to take those sorts of chances.