Without getting too in the weeds. If you're a Microsoft 364 customer already, Teams is likely included at most subscription levels. Adding Slack on top of that seems like an unnecessary cost in that case. Transitioning a culture to use Teams is a bigger challenge. For meetings we've been found a good adoption rate, but for actually making Teams there's less motivation. I personally love the Chat capability, but getting people to respond to emails is hard enough nevermind thowing chat into the mix.
Microsoft 364 was a typo, but I stand by it, since it does go down more than I'd like.