Let's say that Microsoft is having a hard time selling Windows 8 to enterprises, many of which don't want the headaches and expense of retaining their employees to learn the Modern UI. To make Windows Blue more appealing to its sizable corporate user base, Microsoft could make its slick, tile-based interface optional, thereby simplifying the upgrade path for many organizations.
There are several reasons why this is unlikely, however -- including the fact that Windows 7 remains a popular choice among enterprises, many of which are in the process of migrating to the OS. And if you've just migrated to Windows 7, you're not even considering Windows 8 -- Modern UI or not. Furthermore, the Modern UI is the cornerstone of Microsoft's (perhaps misguided) strategy of providing a single UI for its desktop and mobile products. Bottom line: If you make the Modern UI optional on your flagship OS, you're messing with your grand plan for mobile/desktop domination.