"the loss is non-consequential other than extra beer"
Man, you drink a lot :)
You have the luxury to protest in this way because from what I get this big website is an e-commerce website with sales revenue that are incomparable with adsense earnings. So if its actually an e-commerce site (which it should be, otherwise removing adsense makes no sense), then it would be great if you shared some statistics along with the story:
What percentage of your visitors are returning customers?
Has the traffic increased throughout the years?
Has the quality of traffic(viewed as customers) increased?
It's wrong to think that google HAS to provide at least the same amount of traffic or earnings for each consecutive month. Most of you are old enough to know how complicated the web can be. A scenario which might fit to your story can be the following. Your website is having almost the same amount of traffic as it was back in golden age of adsense. Although the customers are mostly returning customers, so they are more comfortable to the design of the website, hence don't click on ads (I don't remember if I ever clicked an ad in ebay, and I visit ebay a lot). Again I'm assuming the website is e-commerce, otherwise this should be listed under extreme/nonsense division, unless you replaced adsense with another company.