I'm also going to make a pretty big assumption, which is that a lot of people using mailing tools that give them stats such as MailChimp do not think about how privacy settings affect their metrics. If I have images disabled by default I will almost certainly never show up in an "opened" metric.

Images off by default is becoming more common.

It's a common assumption, but also a dangerous assumption when the people who you assume have established the proof are the same people who stand to profit a from it being proven.

They certainly haven’t proven its efficacy to the people whose resources are being consumed.