The AR-71 Blackbird remains the fastest manned aircraft ever flown, and was perfect for high altitude reconnaissance over hostile territory prior to the development of unmanned aerial vehicles, or drones. Drones have eliminated the risk of a pilot being shot down. They also have the advantage over manned aircraft of lower altitudes, longer loiter time, and less cost to produce and operate.
It was right to discontinue the SR-71 because, in spite of its impeccable record and stellar performance, it is old and outdated technology. People are too apt to hang on to old and outdated technology out of nostalgia. What is the real purpose for being able to have a jet go that far that fast, unless it can do so on a commercial basis?
The SR-71 Blackbird may be the fastest air-breathed manned aircraft on earth, but it costs around $200,00 to operate the Blackbird for merely an hour. The SR-71 Blackbird's jet fuel also has very low volatility, and needs very complex engineering to ignite it, making it even more expensive. The Blackbird was also created during the Cold War to fight the Soviet Union, so there is no real reason to use it anymore.
The SR-71 Blackbird has had its day, and yes, retiring it was the right thing to do. In its time, it flew high speeds and high altitudes, and was able to outfly missiles. About 12 of the planes were lost to accidents. The funding for the program eventually ended. With unrest in the middle east, and a new types of battle being fought, fighter planes become re-invented with new designs and capabilities. The SR-71 model will be remembered in museum displays at NASA research centres.