Let me clarify by saying I'm not against this hire. My point is that at this time there are several other landing spots that could be considered far more desirable than the Cowboys who could be looking at its last year under Jason Garrett. Aside from the Raiders, there are new coaching staffs being assembled in Detroit, NY, Indy, Arizona, Chicago, and now Tennessee.
Again, you are assuming 2018 is Garrett's last season. The Cowboys have had two back-to-back winning seasons and if the Cowboys have a winning season in 2018, even just 9-7, Jerry will use that as an excuse to keep him.
As a fan, I'm not happy with the team right now. But as a coach, I would rather work for the Cowboys even for one year than go work for several other teams. I don't say that as a Cowboys fan, but just a objective view. Jerry has come out publicly and said he never even considered firing Garrett. The odds of one of those teams you mentioned still having the same coach they are about to hire in 4 years is slim.
Coaches don't think like that. They are looking for position upgrades that help them get to where they ultimately want to be .. head coach and making a lot more money.
For example, let's say you are an assistant quarterback coach for an NFL team serving under a quarterback coach who will likely be there and in that position another 2+ years. You have the option to go to Dallas or any of those teams you mentioned.
In Dallas, you have Prescott who played well in 2016 but regressed some in 2017, so there's room for improvement.
In Detroit you have Stafford who is already a really good quarterback. If you go there and he does well, it's not because of you. If you go there and he does worse, it's all because of you.
In New York, they have an aging Eli Manning who has already won two Super Bowls. If you coach him and he does well, it's definitely not because of you. If he does worse, you get half the blame along with his age. If they draft a quarterback, then your coaching performance will depend heavily on a player other people pick for you that likely won't play well enough to get you noticed and if he does, he'll probably get most of the credit.
In Indianapolis, you have Luck who's returning from injuries and surgeries. If he does well, he was already great. If he does poorly, it's either because of you or he hasn't recovered.
In Arizona, who knows what you'll have to work with there.
In Chicago, you better hope Trubisky plays as well as they expect him to because if he doesn't, it will probably be your fault before his.
In Tennessee, Mariota is already a good quarterback. Only one way to go there and it doesn't end well.
My point is that coaches look at a lot more things than fans do. Fans hate Garrett so fans say, "I wouldn't work for him" but coaches in lower positions don't think like that. They mainly focus on one question .. "Will that job give me the best chance to show my skills as a coach so I can get another offer for a higher up position either with that team or another team in a few years or sooner?"