How did this story eventually end? Did you succeed in changing the culture?
I’m in a very similar situation where the IT department is steering towards Sharepoint, but there is a significant difference to your situation four years ago: Microsoft Teams is part of the picture. They are looking at Sharepoint for file sharing (which seems reasonable when it’s included in the Office 365 package anyway) and at Teams for communication. I’m fine with using Teams, but it’s not suitable for organization wide discussion and knowledge sharing (at least if your org has more than 2500 members, the max. size for a team), which is why I’d like to integrate it with a discourse forum.