There are infinite undocumented “things” integrated with Microsoft solutions. Just of the top of my head, here are couple that i’ve encountered

• SCADA software

• Entire business critical database application written in access

• Hundreds of tailor made order documents for logistics that are made with Excel

• Accounting software that only runs on Windows

• The immense cost of moving all of your projects from the web that is teams/sharepoint/OneDrive

I mean. Someone will have to maintain the same kind of infrastructure that visa and mastercard does. It would require a monumental amount of investment. Not to mention that it can, under no circumstances, go down during peak use.