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How does MS makes more money for windows-10 users than windows-7 users if windows-10 is free ? Have the store revenue became that significant ?

Windows 7 is "free" to the user as well. The manufacturer of your computer will have paid for it. Windows 10 is only a free upgrade.

Windows 10 OEM install licenses still cost money. E.g. Amazon $114

Also, subscriptions to Office 365, Microsoft Drive etc.