system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does Microsoft give Windows 10 away for free to Windows 7 and Windows 8 users?

Edit: Thanks everyone!
gpt
It's part of their different monetization strategy. Rather than selling Operating Systems, they now sell software (or more accuratly subscriptions to software). 

In fact, since the Windows 10 operating system is shared between phones, tablets, holographics ,computers, and even the Xbox, it means Windows get an extremely big market.

So every system that works on any of the above systems, can also work on everything else. Through that, it means that Microsoft will get the dominant marketshare in apps on each of the systems.

By getting everyone on the same system, Microsoft ensures that every developer will want to make titles for the system, and thus, that everyone will switch to it's systems.