I think the biggest hurdle W10 has to overcome is getting people to upgrade from w7 and XP. A lot of people are saying things like "1.5 billion pcs in the world, free windows 10, boom now they got market share". Universal apps can't be a savior if its only on 15% of PCs.
So while OSX is gaining market share that isn't their biggest problem. I really do like W10 and see it as big improvement over W8, but i don't think the changes made are going to win over those who didn't like W8. It will make w8 users happier and some w7 users will upgrade because its free, but i think this is a hail Mary strategy.
W7 will always be the biggest competitor to W10, and ill be interested in the numbers next year when its not free.