There's one common demnominator between Challenger's and Columbia's losses:
Both accidents originated a little after launch, because of e problem occuring outside of the orbiter but directly impacting (figuratively and really) the shuttle.
The events desttroyed the shuttles because the Shuttle has an external tank and boosters located at the same level as the orbiter which is exposing a huge surface to those tanks.

That's why NASA should pay attention to OSP designs that once again involve a shuttle riding piggy-back with its boosters. It might not be a good idea.
Launched vertically, piggy-back, means an escape system is more problematic.
But a capsule, atop a rocket, exposes far less surface to the rocket underneath it.
Not a single capsule, throughout the history of manned space exploration, failed because of its heatshield.
Now, if a winged orbiter could be riding piggy-back above a horizontally taking off airplane-carrier, that would be another matter. But this means the orbiter would have to fly on its own from a speed below mach 1 up top orbit (I've seen several similar concepts involving the Antonov 225 or a modified Boeing 747).

But capsules can also be designed to carry up to 6 or 7 persons, be docked for a very long time to a station or an interplanetary spacecraft.
Capsules could get us much further than LEO, which winged shuttles cannot.
It's only that capsules would involve (politcally oriented) decisions for spaceflight expeditions, that take place in a punctual manner, rather than systematically like with the shuttles.
Capsules could involve a far smaller astronaut corps than today's.

However, if a shuttle is destroyed, it cannot be used anymore. If a capsule is destroyed, another new one is built anyway.

Raoul