While this is generally true, is how I recommend to treat Slack, and how I treat it myself, the reality is that its nature is neither here nor there.

It's true that if you treat it just right, you're going to get an async tool. However, you want tools that are naturally like that. There are many foods I can fairly easily cut with a spoon, yet my experience is much better using a knife.

The mere fact you have to encourage to close Slack means it is used for something it shouldn't. Use evergreen communication for most communication, then use Slack only when you need it, which will be much much less.

Slack is also terrible at history preservation, see my reply to a sibling comment.