There's a lot invested already in just getting the main mass up there and the core development of the system so building and installing small improvements can k

There's a lot invested already in just getting the main mass up there and the core development of the system so building and installing small improvements can keep the investment useful for lower cost than replacing it. Documentation on the spy satellite connection is sketchy but it seems to be designed similarly to an NRO satellite KH11 iirc but I think it was modified a lot and the more modern spy satellites might not be as readily convertible to deep space observation. So you have that work and the cost to loft it vs the cost to install some upgrades to and existing satellite.