I actually find it a little difficult to believe that they would give up that ritual as easily as implied. As far as these people are concerned, the lives of every person on the planet depends upon The Sacrifice. Then some guy comes along and says, "Oh, you know that ritual thing? Yeah, actually, we don't have to do that anymore."
I'd also like to know why the builders of the device didn't make whatever change McKay made to expand the field. I mean, if McKay can think up and implement the idea, why not one of the Ancients?