Those who upgrade from Windows 10 to Windows 11 can go to Windows 10 for 10 days at any time. After that it gets harder.
If you upgrade from Windows 10 to Windows 11, you can override this update at any time for ten days in the Windows 10 Settings menu.
According to these questions
From Microsoft’s OEM Partner MSI, the following applies: After downloading Windows 11, you have ten days to try out Windows 11. You can always switch back to Windows 10 during this period. All files and data you move from Windows 10 to Windows 11 must be retained during downgrade.
After this 10-day period expires, users who want to remove or replace Windows 11 can at least download the Windows 10 ISO file and replace the existing Windows 11 with a completely new installation of Windows 10. However, all the files and data on the system will be overwritten. So before backing up you need to back up all the files and personal data yourself and then reload them on the restored Windows 10 system.
One reason for the revenue is that you do not see the Windows 10 feature that Microsoft removed from Windows 11. For example, live tiles, Cordana, Skype, Onenote or Paint 3D. However, as you upgrade from Windows 10 to Windows 11, applications already available under Windows 10 are also acquired. Read more about it here:
Another reason is that an attractive application does not run on Windows 11. Or you think your computer was faster with Windows 10 than Windows 11. Or a hardware under Windows 11 will not work properly.
If you’re actually going for Windows 10, keep this in mind: Microsoft will end support for Windows 10 in 2025.