the thing that most people don't understand is that Microsoft doesn't make only slack, and their other software is usually pretty good.
Excel for example is a marvel of software engineering, Word is pretty great too. Outlook works well in an enterprise setting (though i wouldn't use it for personal needs). And their groupware works remarkably well. And their ActiveDirectory (Domain controller + LDAP/Kerberos) is fairly good.
And guess what? If you're a 10-people company sure, you can do without Excel/Outlook/Word/ActiveDirectory/etc.
But when your company gets bigger it's just the choice that makes the most sense.
Oh and guess what: most of the software in named is native software and works offline too.
So basically your company is not buying Teams... Your company is buying an office suite and gets Teams for free.
Also... People (developers mostly, I must say) tend to forget they're not the only people on the planet. People from the accounting/hr/logistics/marketing departments are probably just fine with Teams.