Even if you assume there are sufficient working parts in orbit to construct working satellites, doing that reconstruction in space would require gathering them

Even if you assume there are sufficient working parts in orbit to construct working satellites, doing that reconstruction in space would require gathering them together in space either by people (super expensive) or by a sufficiently flexible robot, in which case we're already sending up something, just send up the desired new satellite instead. Alternatively we could deorbit components without them burning up and assemble new ones down here, but that would also be vastly more expensive than just making new ones down here to begin with.