The correct course of action is to have an API that allows other apps to integrate and provide this feature in Office.
To give an example, Google, when adding storage APIs to Android KitKat worked closely with competitors to Google Drive to ensure that the storage API worked with third party providers. Google are not perfect, but that is a fine example of the right general idea.
And yes, O365 adding chat was a big blow for Slack. But I think Teams' displacing Slack has more to do with the fact that Slack doesn't offer value equivalent to it's fairly high price. There are lots of features in O365 that never disrupted "premium priced" competitors - BaseCamp exists despite O365 ToDo, project management tools exist despite O365 Planner, etc.
You claim that Slack doesn’t offer enough value to justify its price, then why did people pay for before? Teams is cheaper, sure. But is that because it’s actually cheaper to operate, or is it because Office is a giant cash cow, and Microsoft are subsidising Teams pricing with Office profits?
Teams is quite clearly a separate product to Word or Excel. Both Teams and Office represent entirely distinct products that are fully featured and can be used completely independently of each other without any real loss of functionality. There’s no reason why they should be bundled, Office was perfectly profitable without Teams for many decades. It not even vaguely the same as Excel adding Python support, Facebook adding chat, or Reddit adding image uploads.
Phone lines have always been part of a functional office. Not sure how you can argue that communications (voice, video, screen share) is not part of a modern, WFH enabled office suite.
And what does comms and phone lines have to do with Offices market dominance? Are you suggesting that Office should come with a mandatory bundled phone line as well?
Smartphones include features that have relegated to niche markets the consumer camera market, consumer dictation equipment, mp3 players, navigation devices, etc.
With regards to Google’s productivity suite vs Office. Googles stuff is very impressive, but honestly not even in the same league as Office. Googles stuff is pretty useless for anything beyond pretty basic editing, and I say that as some that used Googles offerings extensively in a professional environment for many years.