In brief: The South Korean government believes Windows will be too expensive to support in the future, and in a bid to reduce costs the country plans to transition to the open-source Linux. While there will still be costs associated with support and new PCs, it's believed there will be both cost reductions and a reduced dependency on a single operating system.
Earlier this year, Microsoft signaled the end of support for Windows 7, which still powers many machines around the globe. Aside from shelling out serious cash for paid support, most users will have little choice but to migrate to a newer version of Windows.
However, South Korea has opted to change operating systems entirely, perhaps to Microsoft's dismay. The South Korean Ministry of Interior and Safety announced plans to migrate to Linux, starting with a trial of sorts to test for compatibility and security. Assuming there's no mishaps, a broad rollout could commence shortly thereafter.
A government-wide adoption is expected before the expiration of Windows 7 support, currently slated for January 2020.
The switch is being spurred by both the cost to maintain Windows after support for Windows 7 ends, and the desire to reduce dependency on a single operating system. The migration to Linux and the purchase of new machines will cost an estimated 780 billion won ($655 million). These costs are presumably easier to swallow than maintaining Windows machines through paid support until 2023, then having to upgrade to a newer version of Windows.