The forces in Washington working to convince Congress to pass a wildly higher defense budget were pushing Soviet interceptor / fighter superiority very hard. This plane was something they didn't want anyone to know about. The fact they ordered tooling destroyed, a very unusual act, demonstrates how serious they were.
As for practicality, seems they've been flying SR-71 spy planes for some time now. It's basically the same airfraime. Many tricks were developed to overcome the extreme temperature variation during flight.