I would hope they would at least look externally for a candidate and not just promote from within just for the sake of it...
Why?
I don't think they would be opposed to hiring externally, but there are a lot of disadvantages to doing so as well.
External candidates need to learn the personnel, personalities and politics as well as the culture of the franchise. They may have their own systems they want to implement which can cause confusion or interruptions to growth. They're not better just because they're from somewhere else.
Building a coaching staff is no different than building a roster. For your team, you want to build from the draft, draft guys that fit your system and develop them to do what you want. You can sign free agents, but sometimes they don't fit what you're doing or do things their own way. And they're more expensive. Sometimes you have no choice, you have to do it. But if you can develop resources within, it's usually better.
Big Ben had a really difficult transition period to Todd Haley, and while they finally sorted it out after a year and a half, that's two lost seasons for the Steelers who have missed the play-offs both years. And it was the transition to the no huddle which jump-started that offense, which quite frankly, they didn't need Haley for.
The Broncos modified their offense to fit Tebow when he was QB, and then again for Manning. When Mike McCoy left, they promoted internally with Adam Gase, again to ensure no interruptions for the QB.
Daboll could very well be the next coordinator if Josh leaves. I'd rather have a guy who has Brady's trust and comfort level than a big-name OC from somewhere else. Brady's been operating in the same offense more or less since his rookie season. It's transitioned over the years based on personnel but there are still plays he calls from his first year, just with different groupings. And that's a huge part of our success.
This offense has finished in the top 10 in points scored the past 10 seasons, including the top 5 the past 4 seasons. I really can't think of any external guys who I would trust to continue that success.