[rrmm]

Post Columbia accident, most of the shuttle missions were focused on the ISS where the crew could have taken refuge had there been a problem during ascent which prevented safe re-entry.[1]

As you note the Hubble mission (STS-125 with Atlantis) wouldn't have that option available due to its orbit, so NASA had the shuttle Endeavour ready to fly as STS-400 as a contingency.

[1] The plan would have been that crew stranded on the ISS would be picked up by a later shuttle, so NASA was obviously comfortable with the contingency of leaving a crew without transport off for a period of time during an emergency. These were designated STS-3xx missions and would have launched within 40 days of call up. The ISS could have supported the additional crew for 80 days.