Even if they were paying 10$ per user, it's basically the salary of one non-lower level engineer to have cross-company communication they are already used to.
It's the premise of paying your competitors money to do something that you already are doing. Someone manager probably got a bonus out of this move.
And corporate espionage is more prevalent than you may think. Can you absolutely guarantee 100% that the competitor, like Zoom in China, isn't sniffing their video traffic or recordings to some of their most sensitive internal business discussions?
This. Companies are cutting costs. Teams may not be the best to use, but it doesn't make sense to have multiple chat solutions.