funny that it's only available as part of an office 360 bundle. keeps me from giving the likely terrible UI/UX a solid review, too bad ha. they aren't going to compete with slack really, they'll pick up people who have been stuck with skype/lync/messenger for years.

they are leaning on the managers who prefer having one provider of several tools than multiple providers.

How is it ok for huge companies like Microsoft to blatantly copy smaller companies like Slack?

This is Microsoft's MO, from the beginning. When they are most successful, they take something that someone else has already envisioned, and build a slightly better version of it, then bring it to a mass audience. Basic, DOS, Word, Excel, Windows, ad nauseum.