People who are known at time of sending. A slack message can be searched by those joining the team much (much) later, those who move teams, in-house search bots, etc. Mailing lists bridge this gap to some extent, but then you're really not just using email, you're using some kind of external collaboration service. Which undermines the point of "just email".
The idea that we’ve started building enough to bring prices down is a non-starter.