The BBC reports that Microsoft is actively deceiving consumers into switching to Windows 10 (hat tip reneabythe). That's how desperate Redmond is to avoid having to "support" 7. In the screenshot below, when you click the red X (which is normally how you close a popup), you signal that it's OK to install 10 on your machine. This is a classic Dark Pattern design.
Atrios mentions how disruptive installing a new operating system could be to a small business. An "upgrade" requires wiping your drive! It will save your personal files to a folder that's not erased, but programs will have to be reinstalled. If you have, say, dozens of music studio apps with dongle licenses, special connections to drivers, and plugins installed in a certain way, you're looking at days trying to recreate that working environment. This is not something that should happen because you closed a popup.