Even in that case, cheap and unmanned would be much more cost efficient.
The SR-71 was uncontested in the 70'ies, but it wouldn't be today. Today it would be vulnerable. Missiles reach higher and fly faster. There are missiles that are designed to intercept ICBM's, which come in faster than a Mach-3 SR-71.
Radar detection, tracking and firing has also become much more effective in the last 60 years.
The SR-71 was uncontested in the 70'ies, but it wouldn't be today. Today it would be vulnerable. Missiles reach higher and fly faster. There are missiles that are designed to intercept ICBM's, which come in faster than a Mach-3 SR-71.
Radar detection, tracking and firing has also become much more effective in the last 60 years.