Apple and Google are offering their OS' for free, and that's the only reason MS is moving (ever so slowly) towards a free OS business model. A subscription fee would kill them.
That is pure speculation. Microsoft has said they are moving the operating system to a "service" business. They have not stated what that means, yet. A subscription fee is not out of the question, nor are fees for updates, but if they go that route, expect the core operating system source code to be released at around the same time.
The public reasons they gave for offering Windows 10 free has to do with getting more people to develop applications for the operating system and it is the first operating system they will be able to run across all platforms from PC/table to XBox to phone. They want that operating system to be the only operating system they have to support.