As most stated, 60/40 sounds about right. Even though MS made a number of bad decisions leading up to the last launch, the result of the current split is due to Sony not screwing up (launching a year late, significantly more expensive, exotic hardware, etc)
MS did a good job solidifying their userbase in recent years with BC, gamepass, and powerful hardware with the One X. But I don't see any growth for them in the near future if Sony does a safe launch and continue their same strategy.
MS will actually lose marketshare on the console front due to their PC strategy but it will be relatively minor since the PC/Console fanbase are so segregated & MS will likely make up for it with their new revenue streams from PC gamers who would never bought an Xbox to begin with.