So, given that the churches have been doing all this for a long time, I read it as an argument that the current rate of giving is the asymptotic limit for the social technology the churches have been using.

Agreed. I read the initial suggestion as basically 'get atheists up to the levels of religious charitable giving,' which is why I thought it was silly that a response was 'but that might make the religious give more.'

Sure, but in this context we're talking about the social structure of the US (and, in general, Western) society and I'm using the word "religion" in a very conventional meaning.

Sure. I don't think it's that unconventional to refer to, say, UUs as religious, and I expect there to be more secular communities who act like UUs even if they don't self-identify like UUs.