According to the article,

1. They are also migrating backend infrastructure such as emails servers etc.
2. They already have Linux migration experience in some German states as well as the current proposer.
3. Companies such as RedHat and Canonical and OpenSuse do offier enterprise level support. So open source software doesn’t have “after sales” support is a myth.
4. They say that the goal of the migration is privacy and security, no necessarily cost driven. They very well be prepared to pay a premium for enterprise level support.