Couldn't wait any longer, so this is my experience:
- Clicking the Back button just took me to some sort of login screen, which allows to change user names but on Next takes you once more to "The legal stuff" page.
- Clicking the Decline button opened a confirmation dialog window with a title similar to "Aborting free upgrade to Windows 10" and its text along the lines of "here are the things you'll miss out on by not installing Windows 10", like the free Windows 10 upgrade itself, free apps and "increased stability and security". And that reverting back to the previous version of Windows "might take a while".
Which it did. I'll edit this once I know whether files, programs and settings are back to what they were before this waste of the owner's as well as my own time.
All files and programs seem to be where they were before the upgrade, the only curious things that needed to be re-entered were the WiFi password and another password for some cloud software, all other settings appear to be unchanged.
A bit later, while looking whether everything was in order and keeping an eye out for any unusual activity, I found that Windows Update started downloading Windows 10 again!
Trying to disable the relevant entry in the list of Windows updates didn't help, whenever the list was re-opened, the Windows 10 entry was again checked to be installed.
So I resorted to some registry changes mentioned in another very useful answer here which was written for Windows 7 but seems to be working nicely for Windows 8.1 as well.