Microsoft announced back in 2015 that Windows 10 was going to be the last version of Windows. Instead, they were going to release updates to Windows 10 forever (or so). Of course, this approach has always been the case. A new version of an OS is always (almost) built on the last one. Numbering is just marketing.
Now, Microsoft has announced Windows 11. Why? Probably in large part because of macOS 11 with support for Apple Silicon and ARM Macs.
It's been known for years that Apple eventually was going to ditch Intel for its own silicon on the Mac. And with that be able to run the same apps on all their platforms.
Microsoft also just announced that Windows 11 will support Android apps. They also copied the dock :-) Competition at its best.