Thinking about it this isn’t necessarily true in that moving the FOCAL relatively little could yield new things to observe (even microarcseconds). So you wouldn’t need a new FOCAL to measure each new thing. However each FOCAL would be measuring a miniscule bit of space over its lifetime. Which means for each distinct object that isn’t basically a neighbour in angular terms to a FOCAL sent you’d need a new FOCAL probably. Unless our long term energy generation/harvesting and propulsion in deep space significantly improves technology wise.