So the question is where the new Windows 10 growth will come from.
The answer is obviously from the sale of new PCs, to the tune of around 250 million each year, even now (65 million were sold in Q1 2016).
Regarding the reason people did not upgrade, it has nothing to do with neckbeard worries. Regular people are simply scared of an upgrade breaking something and having to learn new things. When they buy a new PC they will not have a choice.
So it's safe to say in a year's time there will be more than half a billion Windows 10 PCs with Redstone 2. There is no mystery or challenge.