Ads pay for free.
Not really. When you buy the products you also pay for the price of their ads. Buying a box of cereals? It will cost you the cereals (+profit) + the price of the ad (+profit of ad agency) + the price of distributing said ad (+Google profit). Yes, they apparently generates more profits and a larger variety of jobs (thus the "economists" will declare them to be "good for the economy"), but they're not free at all.
There will come a time where a company demands users install software on their machines and send every single key tap, mouse click, and any other item they can get access to as individualized ad data.
It's already happening. It's called "Cloud computing".
Where there's a will, there's a relative.