Re: Invalid numbers
I think you are underestimating the numbers hugely for Skype/ teams usage.
I doubt it but I suspect it depends a lot on how you define the use and hence the market. I've had to use Lync/Skype/Teams for one of my customers for years so I'm reasonably familiar with it and its use. You need a VoIP infrastructure to replace a traditional system after which it doesn't really matter what clients you use for it, but, sure Skype now does this quite well.
The market is really what companies are prepared additional money on. I'm not overly enamoured by Slack but I can see why it's popular in some environments and why Microsoft has been prepared to make Teams free go after it. I just don't think it will be enough to convince those with an investment in Slack to jump back. And, as I said, I think Microsoft is likely to miss stuff playing catch up.
When it starts force-feeding teams on GitHub it might be successful in establishing a new standard client but I'm not sure where the upside will be. Even if they achieve a dominant market share they're likely to find it hard to start charging. This has been the case for a lot of Microsoft products over the last ten years.