Tesla started less than a decade ago and has sold out of both the Roadster and the Model S. That's pretty incredible for such a young car company.
I don't think Elon is just "making money on what's hot." Building a car company isn't in fashion, and an energy revolution is a pretty clear vertical right now (see Khosla's invetments). Likewise, building a rocket company isn't very fashionable. If you look at his vision--he's playing a 20-year game--the true market for SpaceX doesn't even exist yet.
I spent some time at Orbital at a time when they were trying to launch ORBCOMM. What they were doing seems similar in feel to SpaceX. I also noted that ORBCOMM is one of the first scheduled launches for SpaceX.
Interestingly, Orbital was started by an entrepreneur with a vision of small launch vehicles and he managed to actually pull it off- it was not some huge spinoff of an existing giant. I wonder how much the two are talking.
Friends of mine who worked on the early days of Orbital's Pegasus described a situation that sounded very startup-like to me. (I worked there in the late 90's, but on Hubble, not Orbital's commercial projects).
Orbital had modest success (if an IPO can be considered modest). So it seems not only plausible, but likely that a new approach is what is needed here. Perhaps SpaceX is the next iteration of a leaner approach that will actually scale to the meet the grand vision.