As a counter-anecdata, I’ve gone three jobs using only Teams - in one case they had explicitly rejected Slack on the basis of price. My daughter also uses Teams for school.
Same anecdata here (EU) - Kids use Teams for distance learning, pretty much every larger company uses teams (and to some extend zoom) due to the Covid situation.
Hardly anyone even knows slack exists. Everyone knows and uses teams.
Hardly anyone even knows slack exists. Everyone knows and uses teams.