But even with if they decide to use services like hosted exchange, ad in the cloud, Office365, etc.
If thats even possible (in-house policy, ...)
Migration will take a long time and the clients have to be replaced first and also will cost a lot of money.
When Microsoft starts hosting in german datacenters it will have more impact on companies who's infrastructure already running mostly MS services and think about moving those to the cloud.
I don't think it's a move against linux and open source software. (at least not a big one)
More of a threat are vendors who only develop software which runs on Windows, require a client that only runs on Windows or need a MS Service(MSSQL,...)