Microsoft can't change the UI too much. Many people will jump to Apple if they have to learn an entirely new UI.
Microsoft also tried to drastically change the Windows UI in Windows 8. Most people hated it because they were running Windows on a laptop or desktop, but the Windows 8 UI was designed for tablets. (Microsoft thought tablets were the future of computing at the time since the iPad was such a big success.)
Microsoft hasn't completed the UI overhaul in Windows 10 yet either. A lot of the advanced options in Settings open up old style Windows apps like Services, Device Manager, etc.