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It is true that most or all of the biggest sites are not affiliates but instead selling ad spots via traffic brokers / ad networks / direct prepay placements. Just try to contact any big site offering to pay $300 PPS or 70% revshare, they will all reply they don't care and only sell at fixed price or via ad network.
However, the many blackhat-ish and blog-ish guys still are affiliates, and always will be, since they can't get adv deals like big sites anyway, or simply have not a site at all but instead use tricky methods they can't put normal ad networks into.
Also in past it was always revshare, now the pay per free signup, CPA, is popularised.
About the % of affiliate VerSus media buy it depends by 1) type of program, for example cams or dating or paysite or else, and type of pay like revshare, pps, per free and 2) how the program is old, new programs can not have many affiliates yet; and 3) is that a period of media buy or not? As example let's have the program buying lots of ads a few months making x3 sales, and next few months no ads buy, left only organic + affiliates: then affiliates % will be x3 what it was few months before when media buy it was strong.
In our case, as a cam site, without affiliates it would be quite painful for sure, since media buys at times convert very bad - also it cost a lot of time for us in managing campaign while runs, to check performance of every advert and removing what does bad etc., time is money too. I remember while running lots of adverts in multiple networks for a launch, no one here could do anything else than check those ads, the time required to tune it was massive and 24/7. We gladly leave this to affiliates, even paying an extra, worth the time.
Actually lots of fraud affiliates (esp. for pay per free) take a lot of time to check and bust too, but after a year or two you collect several good affiliates which stick, and it is sure worth, on long term.

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