I've used Slack calls occasionally, and they generally seem to work well—but have you found Slack calls good enough to replace Zoom?
I think the problem with Slack at least for us, is that its way too easy to jump on a call and we fear people might use this in a way that is counter-productive. We stick with Zoom meeting that are 90% scheduled ahead of time, so everyone can have their "focus time"
Slack calls work great for 1-1 calls in my experience, but don't tend to have as good of quality as Zoom calls on poor internet connections. Additionally, Slack only lets you include up to 15 people in a video call. That makes Zoom better for larger team calls, or for calls that may be over less reliable internet connections.
My default is to use Slack for quick, one-off calls, and Zoom for larger, scheduled meetings.