It's not about being bug free. They may have the same functions, but they require different setups and who knows if you'll be able to move your messages from one application to another with no issues (or even move to different PCs without issues). This would also mean that MS is taking a risk with transmitting sensitive data on applications they don't control.
You are making the dangerous assumption that both Slack and Team have 0 bugs and/or will not have problems in the future. Internal usage of your own tools also helps with their development.
I have no idea why you are so gung-ho against this. I think it is time you removed your tinted glasses and look at both the cons and pros of MS limiting the internal software stack from an objective point of view. There is such a thing as being too flexible.
I'm sorry I've given you the impression I'm being "inflexible". I'm not.
I also used the precise phrasing,
"assuming the programs are interoperable". Perhaps they are not. In which case the program which can be rewritten on site, (Teams), is the better choice. I honestly don't how a concept that obvious, drew such a pedantic lecture stating the very same thing, but in a rather more long winded version.
However, I do know the people above the age of reason, don't do anything, which doesn't have an ulterior motive.
For example, M$ says, "Windows 10 is our best (and last) operating system ever". They also say, "Windows has progressed.to a service", which kinda smacks of a prelude to Windows becoming a "subscription"
What Windows (10) has become, is a vehicle to herd customers to the "M$ Store", pretty much like cattle being prodded onto a boxcar, while culling their personal information in the process.
M$ is also doing the same thing to customers, as it is doing to its employees, by using Windows as a tool to render much 3rd party software useless. Except now, they've taken that to a whole new level, by announcing ahead of time, whose software they intend to break with their next update.
As to ulterior motives, the motive behind the edict may be a simple one of concern how people's perception of M$' software would be affected as a whole. "If it gets out, (which it obviously has), that our employees would rather use another company's software than our own, we'll lose prestige, as well as faith and respect from our customers and the rest of the industry as a whole, and god knows we can't have that"!
So, I don't know if this is an issue with you being a M$ employee, my syntax, or a slight "English as a 2nd language misunderstanding", but I'm certainly not "gung-ho" on either company's product. Nor do I believe I've posted anything to that effect.
In point of fact, I've posted in a humorous manner, borrowing the derisive term "slacker" from colloquial English, and suggesting that M$ "pump Teams, full of emojis" to level the playing field.
So really, if anybody has blinders on, it's not me.