I think now, they certainly are. But back in the Sixties and Seventies, they weren’t.
When you needed an asset over an area for surveillance...you were dependent on the timing and orbits of satellites. Only the very high end satellites had any maneuver capability that could have been considered for non-emergent use.
Of course today, it is different. Satellite coverage is nearly ubiquitous today, so that likely explains why we retired ours.
Let’s say Mig-25 and especially Mig-31 weren’t near that expensive and not near as dangerous to fly but really close in specs. The latter is the reason why SR-71 missions were scaled down. And they build thousands of them.
The Soviets had practical ion engines allowing the light spacecraft traverse the orbit the way they wanted by 1972.