It has "private" group chats in Chats tab and all meetings auto-create group chats. At various points I've tried to push conversations into a proper Team and threaded chat, but the "convenience" of just sending a chat message in last week's meeting chat or some hand-built group chat rather than just finding the right Team channel seems to keep winning out more often than not.

Are you talking about threads or channels? It sounds like you are describing what slack calls channels.

Yes, in the places I've used Slack, channels were the default mode of communication.

In chat, just go to New Chat and add as many people as you want. Boom, unthreaded group chat. You can rename it to a topic or group name.

But then what's the point of the "Teams" part of Teams?