Quote from docs:
"The operating system will use the available free memory for filesystem cache, which allows the compressed MongoDB data files to stay in memory."

We need an instrument to reduce memory (RAM) usage of whole MongoDB subsystem because we should start several services on server. At this moment we can limit only WiredTiger internal cache (storage.wiredTiger.engineConfig.cacheSizeGB config setting).
For windows users the only one way to limit entire instance memory usage - to create virtual machine for MongoDB.
Why don't you add configuration setting to limit entire RAM usage?
Do not offer OS tools.