I think the issue is less about building such a rocket system, and more about actually testing and certifying it to NASA's modern standards for carrying humans.

I think the issue is less about building such a rocket system, and more about actually testing and certifying it to NASA's modern standards for carrying humans. See the constantly stretching timeline for developing the Crew version of the Dragon capsule, despite the fact that SpaceX already had a working and tested rocket and pressurize capsule before they started on the crew version. I'm optimistic that SpaceX can make some rapid progress here, but I wouldn't bet on the timelines holding for any of these human spaceflight projects.