The most likely reason for the letter IMO is simply the recent news all around us about the increase of opioid related deaths to above 100K per year. The last sentence in the letter refers to Doc's "perspective" and "views" and that is exactly what I believe Doc wrote about.

https://www.cdc.gov/nchs/pressroom/nchs_press...211117.htm

https://www.ama-assn.org/system/files/issue-b...erdose.pdf

https://www.usnews.com/news/health-news/artic...first-time

https://www.forbes.com/sites/brucelee/2021/11...sing-news/