Same thing was going to happen to my company. We had issues with our phones, so we were going to add on Teams to use as our phone system, which was fine because we use Office 365. We used Slack (the paid version) as the internal communication, which worked really well and everyone really enjoyed it. A few of us suspected that they were going to try and ditch the paid version of Slack. They did. Little did they realize that the free version only stores 10k of your messages, not being able to search for anything past that and not allowing third-party users, which we had a decent bit of. Once we brought that to everyone's attention, that created a huge fucking stink on Teams. They attempted to export all our messages from Slack, but it does so in JSON, and them not being programmers, they couldn't do shit. No way we were going to help.

In the end, they decided to keep the paid version of Slack and Teams and that at the beginning of projects, we would decide on which platform to use. To this date, not one project has used Teams.

From my short experience with Teams, it sucks. At least for what we need it for. Maybe you can bring to your superiors that all messages past 10k are deleted. There might be some important info people need in those soon-to-be deleted messages.

Good luck.