While there are definite problems with how ratings are determined the ratings determine the ad cost. I’ll certainly never defend Neilsen, their system was antiquated the minute the first home VCR shipped, but their numbers are the numbers that are used so no matter how little they reflect real world viewers they are the numbers that legitimately decide which shows stay and which go.
But that IS the point these decisions are based on numbers that are at best a "guess" which is more than absurd in this day and age.
Any Desktop recording unit like TIVOs and DVRs as well as Cable Boxes (non record/time-shifting ones) and Sat Boxes have the capability to accurately measure Exactly what is shown on the TV screen. And such a system would eliminate much of the Bias built into Nielsen's system. But inertia STOPS the usage of this new technology (also there has been a move in the minority communities to stop a changeover to the more accurate system being that Nielsen over samples many of the minority communities to escape criticism.)
Basically the Nielsen system is akin to a cartel controlling exactly who gets the goodies. Its far from the "free Market" because the scoring methods are faulty and in some cases down right biased.