It seems as well that Microsoft wanted the locked-down environment to prevent Windows RT from having viruses,
I don't think so.
Microsoft, ultimately wanted to duplicate Apple's App Store Environment. They were hoping the lower price point would bring in the users, which would spur development of the Applications for it, which would of course induce more to join the ecosystem. Once Microsoft realized the value of the entire system, they were willing to try and duplicate it.
Of course, the hardware was there, but the Apps and the OS itself fell short, and they were not able to complete the task at hand. In order for them to have a chance at success here, they need more time. Time that just may not be available.