The premise of the question is that direct messages are somehow different and require a separate application. I don't think that's true. Slack and Teams do an absolutely fine job at 1:1 messages.
I’ve been apart of organizations that use Slack and the proliferation of channels is insane because they are so easy to create.
For small-scale groups, Group DMs are fine, though, I would warn to try to use Group DMs only for when more than two participants should get an urgent ping on a message. (Users can "mute" group DMs so they don't get notified by them.)
Group DMs can also have usability problems, and they tend to clog up the UI of slack. I currently have 5 or 6 group DMs going that all have at least one of the same people in all of them, so now when I use the quick switcher I see all of those. Some of them even have 2 of the same people in them - it makes it really easy to select the wrong group.