Re: Some assembly required
This was my thought too; SpaceX could concentrate on making the regular Falcon a man-rated booster for putting a small capsule into LEO. There's already a market for this with ISS missions.
If you want to go further into space you can do an unmanned launch of your deep space vehicle on a heavy or ultra-heavy booster and then send the crew up on your Falcon capsule once you're happy that your Mars vehicle (or whatever) is up there and checked out.
The heavy and ultra-heavy boosters are never going to be as well tested as the regular Falcons, because the market won't support the number of launches needed, so it doesn't make sense to risk crews on them when the overall cost of a modular approach will be less anyway.