I'm seeing a lot of vitriol in this thread, but I don't think it's accomplishing much.
Think about the situation from Microsoft's perspective. Change is expensive. We're not competing for their attention just with those who actively enjoy the new emoji, but also with those who view them ambivalently. And Microsoft has a lot of customers.
Further, anyone who is this angry is probably going to hate Microsoft whether they change the emojis back or not. Microsoft probably doesn't see you as profitable to win over. Catering to those who hate your product is a very bad idea.
Beyond that, Microsoft has stated ideological reasons for the change, and people are willing to undergo cost to promote ideas. They probably view us as ideological opponents since we're opposing their efforts. They will change the emoji back only if they decide not to promote "emotional honesty and playfulness" (by their definition), decide that the new emojis are an ineffective means of doing so, or see such huge potential financial loss that they decide to sacrifice their ideals for money.
And the very fact that we're complaining indicates that we probably can't just switch to Slack or we would have done so already and moved on with our lives.
I very much sympathize. The current Teams emoji feel like SnapChat. But if you hate them that much, then vote with your money and use a different program. You don't have much power to change Microsoft's mind (and raging probably diminishes that power unless you can convince others by doing so). Improve your own lifestyle instead. If you can't switch off Teams, you're pretty much powerless. Either deal with it or learn to communicate without emoji. Leave feedback, sure, but don't give yourself a heart attack. Communicate and prioritize for your best lifestyle. Change what you can change rather than taking responsibility for the decisions of others.