I don't think it is mind blowing nor do I think it will impact their bottom line.Giving it away for free is mind blowing and will really impact their bottom line. Unlike Apple, they rely on the sales of Windows. Apple gets their money by selling you the hardware. MS doesn't really do hardware (other then the xbox/mice/surface tablet)
The damage has already been done by Windows 8. There is a pent-up demand for Windows systems that has been waiting for whatever-is-beyond-Windows-8.
Giving Windows 10 away for free will help move the existing Windows 8 devices up to 10. All new systems will ship with Win10 so the MS tax will be collected on those.