These are the items which editors have selected as connected to this topic. One of the most important factors is that of externalities.
Money spent on marketing one organization can convince donors to give less to other organizations.
As I'm sure you are aware, the percent of income that donors give away has been pretty flat over that last few decades. So practically speaking, it's a fixed pie. Unfortunately, while you admit the existence of these sorts of practical considerations in the middle of the paper, you offer no reasons to think they are not important and completely ignore them when drawing your conclusions. When one organization expands its campaign, competing organizations must spend more just to stay in the same place. In the limit, competing campaigns may cancel each other out in their entirety rather than expand the pool of total donations.