AFAIK, one primary reason for this drop is - automation.
I subscribe to a lot of newsletters, beta-lists, product updates EVERY DAY. (trust me, a lot)
As a reader/consumer, I've always loved conversational emails but tend to avoid brands or newsletters where I can clearly see the automation. Maybe we should take some cues from the top earning "affiliate marketers", they sell one product at a time and write in a conversational form. (Example below)
It's not only the subject line that matters now, you wouldn't open a brand's newsletter every day just because it has a "catchy" subject line. We need good content inside. Subscribers are more likely to open your email if they found the previous one interesting.