> "This approach sounds much harder for an ad network to pull off and sounds like it would add a lot of risk and complication.

Harder to pull off than advertising at people who might actually want to see the ads? More risk and complication than the growing backlash against advertising in general entirely because of shady advertising practices? More risk and complication than having to keep track of various countries' and states' laws re; privacy?

> "For example, what if a web master decides they don't want to have ads on their site anymore. Whoever just paid for that space gets screwed."

Existing contract law already covers this in most places. If you paid for ads to be displayed for a certain time period and they are not, then there's been a contract violation.

This can all be sorted by contracts? The ad network pays out only after the ad has fully ran for 7 days, and if the webmaster removes the ad or similar they don't get paid and the advertiser gets refunded. Enforcing this is trivial by the ad network or a neutral third-party scraping the websites running the ads to confirm the ads are displayed properly.