Twice yearly updates and 18 months between major "releases"? It sounds like Ubuntu, except Ubuntu also has 5 years support on the LTS releases.
Major software projects that have gone to a successful timed release system find that there is much less chance of pushing out dodgy code than there was with a feature based release system. With a feature based release, the code tends to go out whether it's ready or not because the next opportunity may not be for another 5 years. With a timed release system, if a feature is not ready it gets held back until the next release window, which is often no more than 6 months away.
If Microsoft are really going to a timed release system, they will need to overhaul their software development, management, and marketing processes in a very major way. This will not be an easy task for a bureaucracy as large and entrenched as Microsoft's. I won't be surprised to see them fail at it.