There's a lot of risk for new nodes and less for old ones. Exactly how much, I'm not sure.

I guess let me put it this way - I think it's entirely rational for them to be cautious. There's currently not a lot of upside to being more open, so it's usually not worth it even if the risk is small. Programs like this are great because they hit both sides of the equation: increasing demand for open PDKs while lowing the perceived risk by building a track record of successful releases.