One thing to keep in mind is that there's a security angle to this as well. It is beneficial for a company to centralize all of its communication through one service which can be audited for security and also support all the features required to meet the internal Requirements. Having multiple tools multiplies the work needed to maintain security standards.

Of course, I'm not saying that teams is more secure than competing products or that this will automatically improve security. The point is that it's a single option that's easier to manage.

Outside of security, this also applies to cooperation between MS and Github teams. Teams helps with keeping common calendars, meeting invites, recordings and transcripts. Having more tools means that an adapter needs to exist between them and might be disincentivising communication, since engineers would prefer to work on their tasks rather than work around fixing soft problems.