Teams is definitely a solid success. It is by no means a good app. Those two things aren't the same.

Slack started with an aggressive "bottom up" approach, they made something actually good and got to worrying about the sales part later. You don't need sales as much when companies come to you, begging you for an actual contract that fulfills their enterprise requirements, knowing that rooting you out is almost impossible.

Teams went the other way, in typical Microsoft style. Microsoft sells it bundled with all the other Microsoft things it sells. Most companies want a Microsoft contract anyway, and have an established sales relationship with MS, so adopting Teams is a lot less compliance, integration and procurement work than adopting anything else. You don't need good UI if your sales strategy isn't predicated on users choosing you for UI.

And then there's Discord, which really isn't a bad work comms app if you're small enough not to need the compliance stuff. It gives you almost everything the big apps do for free, including unlimited calls, an advanced RBAC system, as many channels / messages as you want, a decent bot API (including media streaming), good notification management, multi-server / cross-organization support etc. They're actively disinterested in selling to businesses (which is what makes them so good, the features they paywall are the features needed by gamers, not serious professionals), but that also means you'll need to eventually migrate off of it when compliance requirements set in.