I think that Google poses a threat to
some affiliates but certainly not all. Everybody has their strengths and Google is the world's best search engine but it is not the be all and end all of everything on the Internet.
The large high quality affiliates are
safe from Google, the likes of moneysupermarket, gocompare, tripadvisor, moneysavingexpert, holidaywatchdog etc.
The big sites spend a fortune on Google PPC but they also advertise elsewhere and they have a recognised brand name and they are not totally dependent on Google. The expertise that they have in their industry and the systems that they have developed over the years, Google can't compete with that and I don't think they will try. They will just carry on taking the PPC money.
The same goes for the large content sites / user review sites. Google crawls information but they do not create content. They
need to show these sites in their SERPs because their search engine would look bad if they didn't.
Small affiliates are both helped by and threatened by Google at the same time because Google holds so much power over them. The small affiliate can create something like say, a 50 page site with useful, detailed information around a single product. This would be unique and should do well at
SEO and have a good quality score at PPC, and there the problems start.
They have no control over search engine results and can drop out at any time. They better have other sources of traffic to survive. PPC can work until the quality score axe falls. Because they are not a "premium" affiliate site with an account manager and spending 1 million pounds a month on PPC their protests will receive standard responses.
So, I think that Google is a not threat to specialist, high quality affiliates above a certain size, everybody else - be careful!