If I can throw in my $0.02 CDN in here…
Their branding and popularity rely on the fact that it’s FREE and it’s RELEVANT. People are tuning out interruption advertising, and Google offers just that, no interruptions.
If they were to do pay for inclusion, they would lose the branding and trust they’ve acquired thus far.
I agree with the Prof that their cash cow is currently AdWords and AdSense. Whatever extra money they make will come from “side dishes to the main course”. I.E. Text ads, APIs, News notification services (just theorizing) etc.
Also the other strong brand association is that Google is banner free. I doubt they would ever move towards a graphical advertising venue. Part of their brand is their visual ‘cleanliness’. The graphical clutter would just destroy it.
In the immediate future, I don’t foresee Google doing any sort of paid service for their organic results.
It never spent a penny to promote its search service … Think about it.
I could really go on and on and on on this, but I’ll spare yall 😎