Actually, different publishers would set different thresholds. If you set your threshold too high, you would get fewer ads. That would help keep the publishers honest.
Since there are alternatives to Adsense -- especially at low CPC's -- this would have the effect of reducing Adsense ad slot inventory by some unknown amount.
As for advertisers, it would surely reduce the inventory at the lower end of the CPC scale. Partly by that effect and partly by the increased competition to display an ad, it would raise the average bid. That, of course, would reduce the number of ads bought.
However, allowing such thresholds would enable sites that currently can make more money with other ads than with Adsense ads to profitably display _some_ Adsense ads, increasing the ad slot inventory.
Depending on which of these effects dominates, it is possible that Google would actually make more money.
Anyway, it would be an interesting experiment, but I'm not holding my breath.