I think that it's most likely they want everyone on the latest OSX for the same reasons that it's good to have everyone on the latest iOS.
Whatever they're planning, getting everyone on board is obviously an important part of that.
I think the biggest thing over the last two keynotes is the iWork update - MS have been ridicuslously slow in the one area they could have won, which was bringing Office to iOS. I'm pretty sure this is the nail in the coffin.