I'd say that most airlines, and most passengers who had splurged on a first class fare for that matter, wouldn't be too impressed with large numbers of passengers getting first class service at the business class fare (know it does happen with upgrades etc).
I know I'd feel slightly ripped off if I was the one person who paid a first class fare (not that it's going to happen anytime soon
) and the 19 other people in the hypothetical who get to sit in first class had paid for a business fare! The airline may as well only offer business only, lose a couple of thousand in revenue from me buying business instead of first, but save a heap more by scaling back the service for those other 19 passengers.