> one more known source of risk to eliminate.
How could they have eliminated that risk?
The risk couldn't have been entirely eliminated, but most likely the external tank insulation could have been modified to at least reduce the risk of chunks breaking loose and damaging the thermal tiles during launch.
We can look at what NASA did after the Columbia disaster; namely, redesign the external tank, employ stricter quality control of the foam across the board, better monitoring of the heat shield integrity, and adding contingencies for being stuck in space with a damaged shuttle.
- They replaced the specific foam insulation that struck Columbia with external heaters, and redesigned other areas where foam was necessary to ensure greater structural stability + minimize damage to the shuttle in case of breakage. They also began more thorough inspection of any heat shield panels that would be reused between missions
- They added various cameras, both on the shuttle and on the ground, to monitor the heat shield throughout launch, plus accelerometers and temperature sensors. Also, the heat shield was checked manually on every mission once in orbit for damage, both with an extension to the Canadarm, and with ISS cameras when possible (a funky maneuver [0] where they would do a backflip to flash the shuttle's belly at the ISS for it to take high res pictures)
- Every mission from then on had a backup plan in case the shuttle wasn't in a state to return to Earth (this wasn't really the case before then, which is kinda wild). Another shuttle was always ready to launch, with a new configuration of seats to allow for sufficient crew space
- They sent up equipment and materials for repairs in space with every launch, though admittedly the usefulness of that was dubious and the repair kits were never used
Perhaps 'eliminate' was too strong a word, but there's no reason these precautions couldn't or shouldn't have been taken before it resulted in deaths and the loss of a spacecraft. (well, other than the aforementioned funding/politics/organizational failure)
[0] https://en.wikipedia.org/wiki/Rendezvous_pitch_maneuver