The thing is, Teams integrates with the rest of Office 365 much more than IE did with Windows. It's very easy to argue that it's a core part of the product.
In general, Microsoft has really nailed how if things work well together, they become more than the sum of their parts. That's why each individual part doesn't need to be the best in class in order to win. Which is bad news for Slack, but it's hard to argue that it's bad for consumers.
Well, I use Ubuntu. No problem with my two customers using Slack (one is Mac basede, the other one is more or less BYOD). All of us install the desktop app and it's done. If they switch to a more integrated system like Team maybe (and only maybe) it could be difficult for me to keep using Ubuntu when working with them. So too much can be bad for some consumer.
Pretty much all of Office 365 is available through a single unified API. Whether they use that API for their internal integrations, I couldn't say. But I would guess any integration they're doing could also be done by a third-party who was that way inclined, yes.
In general, Microsoft has really nailed how if things work well together, they become more than the sum of their parts. That's why each individual part doesn't need to be the best in class in order to win. Which is bad news for Slack, but it's hard to argue that it's bad for consumers.