I think you're right to suggest the actual income from PIR was predictable. But it was also unusual. Deriving so much money from a single place imperils tax status as a 501(c) I am told. perhaps this wasn't a problem for them. (I've never been aware of the board and its finance decisions)
I was led to believe moving to a trust with oversight of a fund is different, and more beneficial as a perpetuating entity. And, Org and PIR relates to the status of Verisign and COM and also places ISOC in a relationship with ICANN which I suspect was irksome, because in so many other senses ISOC felt it was not subject to ICANN direction but actually one of the peer-set.
If I had been on the board, and a well structured proposal had been put to me to do this, I truly don't know what I would have said. My instinct is: this should be discussed publicly and we should get community consensus, but I could imagine other pressures/tensions emerged.
