Ratings still matter, because while the studios and networks can make serious bank off some shows on the back end, it's still the back end, which means delayed returns. They still make the upfront money from commercial ad rates, even on basic cable since they have commercials too. Until the studios and networks figure out a way to make upfront money from a source other than broadcast TV, they are sticking with the old model for the time being.
Also, while a show like The Simpsons has made billions front and back, it struck lightning in a bottle. Many shows aren't popular enough see that kind of perennial return in syndication, DVDs and digital back end.
A classic, pedigreed show like Hill Street Blues isn't even available on DVD, because the first two season boxes didn't sell well enough and the distributor gave up. They've just been sitting on the other 5 seasons until they can figure out a more profitable way to release it all. You can only see 3 seasons of it on Hulu.