There's a great graphic I've seen in the past but I can't find that illustrates why there's nothing we currently have that can service Hubble. Hubble is 150 miles higher in altitude than the ISS. The closest thing we could put together would be a Soyuz, Dragon, or Boeing capsule attached to a Centaur stage a la Gemini Agena, but none of these combinations currently exist in any real sense. While we could theoretically glue enough rocket pieces together to perform missions the Space Shuttle (or Apollo and Gemini Agena for that matter) could accomplish, the fact is we do not have those capabilities ready to fly. That is the advantage the Space Shuttle had. It had both the capabilities and the sign off to perform these missions.