The Blackbird and it's Lockheed sibling the A-12 were designed for speed and altitude goals in order to keep them from being shot down. Advances in anti-aircraft technology forced the change in thinking from High And Fast to the then new area of Low Observability. Plus, low observability usually means small, slow, and quiet. That means the craft doesn't encounter the structural challenges of high altitude or supersonic flight. That means less expensive on a per-unit basis. That, plus advances in camera and data transmission technology, give engineers and designers the ability to scale down the craft to the point that you can deploy hundreds of them for the cost of a single HA/HS craft. Or thousands of them vs an orbital observation platform.
As much as I love the Blackbird, both out of respect the incredible achievement it was for Lockheed as well as it's proven service record, I think the era of brute-force aerial observation is a thing of the past. Sad, too, as they are truly unique and incredibly neat aircraft.
Brad