Yeah... making more free space... like we're going to believe that's the reason. We all know Windows 10 has more malware than any other version of Windows. Microsoft wants control over everything you do. They can't get that kind of control over Windows 7 users, and they know Windows 8 has never been popular. So, getting everyone on Windows 10 is key to installing the inevitable updates that will turn Windows 10 into a walled garden. These restrictions won't mean much if users are able to easily switch operating systems, hence making it hard to switch back to older versions of Windows, and deleting GNU/Linux partitions.
If Microsoft really cared about your free space, they would consider slimming Windows down, so that the OS doesn't need 20 GB of disk space. As for the space taken up by Windows 7/8, Microsoft could put in a button that makes it easy for users to manually delete one's old Windows 7/8 installation themselves.
At the very least, they could avoid downloading Windows 10 on the machines of users that don't want 10. That would save users plenty of space.