Yes, "destroyed" is apt. See the graph under "Slack vs Microsoft Teams: Users" here: https://www.businessofapps.com/data/slack-statistics/
Interesting, but isn't it just because Teams is bundled and integrated with MS Office? I couldn't find any specific stats on revenue or how many businesses actually choose to pay for Teams specifically.
Yes, AFAIK you can't even buy Teams separately, it's only bundled with MS 365. The DAU numbers, however, correspond to people actually online and using Teams for communications, so it doesn't really matter (if we only care about "how many users are using what").