The monetization model would indeed be complicated. But, as an off the cuff estimate, there are several billion Windows installations in the world. A small fee

The monetization model would indeed be complicated. But, as an off the cuff estimate, there are several billion Windows installations in the world. A small fee (say $2/year on average) would easily cover an excellent engineering operation to maintain Windows. And appropriate regulation could force everyone to use Windows or a similarly independent OS, and the whole industry could survive this.