That's a really good point, and it makes a lot of sense. In an ideal world, I'd almost prefer someone who has proven to be an excellent coordinator and a terrible HC: the Gregg Williams types. But then you look at how Norv Turner got another head coaching offer in San Diego, after all of his repeated failures elsewhere, and maybe that just doesn't exist. No matter how bad someone's head coaching track record is, if he coordinates well enough maybe someone will always be willing to roll the dice on him somewhere.
OTOH, even if you promote from within, if your guy shows particular promise he'll get a head coaching job pretty quickly, especially if he comes up with the Patriots. Look at Mangini: we had him as DC for all of one year before the Jets grabbed him. Of course, he definitely left too early, but even if he hadn't, he probably would have left (under better terms) the following year. Following 2007, teams were seriously interested in McDaniels, after a mere 3 years of playcalling and 2 years of being the official coordinator. Ultimately, I think that anyone who succeeds greatly as a coordinator and wants to be a HC will get that offer. You're probably right, though, that that's more consistently the case with imported veteran coaches.