you said...

"Since ICG will be making most of its revenue from people who have an interest in maximizing the price they get for a chip, ICG has a bias to grade higher. After all if ICG keeps giving chips low grades, sellers will stop sending chips to be slabbed."

I can somewhat agree, EXCEPT, that from what I see regarding GRADING in most of the collectibles area is that the Grading Companies have a vested interest in keeping the population of THE HIGHEST GRADE POSSIBLE items in the scarce to rare category regardless of how many items were originally made.

The rarer the "CS70" items there are creates more business for them as "CUSTOMERS will keep sending them their items which "THEY" believe to be candidates for that all elusive "PERFECT SPECIMEN".

I'd bet (pun intended), that if one took a rack of 50 chips straight from the manufacturer and sent them to a slabber that while the majority might might receive a "CS69" only one, if any, would receive the "PRESTIGIOUS CS70".