Its like forced conversion. It only benefits MSFT. Teams get to claim github as a customer, and MSFT saves some money. This might be small, but usually this opens the door to a whole suite of software being shoved down github's work/tech stack.
Video-conferencing wise, I always have more problems with slack than with zoom/teams.
My team also used Slack when I joined and were eventually required to move to Teams. We aren't forbidden from using Slack, I'm in multiple Slacks collaborating with various OSS projects and foundations every day.
collaborating with anyone at ms already meant you were using teams to some extent.
(former github developer)