"what would entice an ISP to follow the RIAA's 'suggestions'?"
Very simple. ISP's put up with P2P users if they have lots of unused bandwidth because the extra money helps. But when things become congested, cutting of the P2P users increases profits and customer satisfaction. This is because the cost of the bandwidth used by a P2P user exceeds the amount they are paying and satisfied customers reduces churn (and therefore increases profits).
Net neutrallity will probably pass under Obama preventing ISP's from kicking P2P users off, so a more likely response from ISP's will be to use bandwidth quotas and charge for going over the quota. That will turn P2P users into a attractive profit center. It has the added benefit of indirectly making money off high bandwith services like NetFlixs. Most major ISP's are already testing this model in limited areas of the country. It's coming.