In an answer to Why does the government still fund SLS while SpaceX is cheaper and has the same capabilities if not better?, Barry Jenekuns said:
Dragon isn't comparable in capabilities to Orion and couldn't replace it without again, significant modification...
What exactly are the capabilities that Orion has that Dragon-V2 does not?
Orion is larger, has a life support rated for a few weeks, has a service module that will allow it real control around the Moon, and is rated for a return from the Moon.
The closest direct comparison is the proposed Gray Dragon mission. This would carry two astronauts around the Moon and back, with no ability to orbit when it got there. It would require the utmost fullest capability of Falcon Heavy to achieve this goal. This would require human rating the Falcon Heavy, which would be challenging, but doable. Also required would be human rating the return system of Crew Dragon from the Moon, which has not been done.
Orion, on the other hand, can support a crew of four, and still has sufficient ability to maneuver when around the Moon. It can support a longer mission, and has proven its heat shield from something close to returning from the Moon.
They are in the same ball park, and in theory an upgraded Crew Dragon could partially replace it. But with the exception of Starship, no other rocket/crew capsule has the capability of Orion/SLS, and Starship is far from launching crew to the Moon.
