Considering how much they are probably spending in order to make this happen (considering the amount of work being done to it), I feel like it was started as a

Considering how much they are probably spending in order to make this happen (considering the amount of work being done to it), I feel like it was started as a proposal to the head people that there was a group of people that normally wouldn't use windows, but could be convinced to use it by having this subsystem. Along with this, supporting open source makes Microsoft look good. But, it wasn't anywhere close to enough of an importance for Microsoft to abandon their data collection system just to go "all the way" with the whole open source thing. It seems like their privacy model is more important to them than whatever an open source model could bring them.