Time will tell but when was the last time any of us got anything for free that did not have some type of strings attached. 
I think there are 2 things that made this OS free. One, microsoft has taken a beating in the rep department with windows 8. Many of the buyers went back to windows 7 and left a lot to be desired with microsoft in there ratings of the then new system. Even with the winows 8 upgrade to 8.1 or whatever it was still badly received. So making windows 10 free helps reverse the negativity.
Second, now that they have all their systems lined up to work together much better(pc, tablet, phones) they will be much more effective in selling their apps. For example, VLC is free. Just go to the VLC website. OR, you can go to the app store and pay about $5 or so. Can't remember the exact price, but Tina made mention of that after upgrading to windows 10 and wanting VLC reloaded onto her computer.
She told me of more apps that are in the app store that cost money, but are free on the apps own website outside the store. Basically, for the "lazy buyers" there is a lot of money to be made on the app store. And now that they have thrown out(at least to some extent) the media player and are pushing the Groove app that is their own version of itunes...the money will be coming in.
For those who are willing to look into things before downloading, the costs won't be going up. But, there are millions in this world that just take the easy, more expensive route, and those people will be lining Gates bank accounts nicely.