The problems with hiring key in that situation are as follows.
1. A new AD. Not just because he or she will want to make their own hire, but because the firing mid year of an AD sends the message of a complete cleaning out. Promoting from within after doesn't just make it harder to attract an AD, it doesn't go in line with the message sent after making the decision to fire TStan mid year. Maybe he was ordered to fire Collins and refused, but I can't see that happen (either the order or the refusal).
2. Key will be judged not just on 8 games, but on his entire resume, including the last 3 and a half years. And if you hire Key, you are likely committing to more or less the same coaching staff with one or two additions. Fans love to pretend that the head coach is entirely responsible for every aspect, but Collins wasn't the only issue we had, and if we turn it around it won't be due only to Key's effort either. That again goes against the point above.
3. The UGA game is the last game of the year, and will likely be a beatdown. I can't see doing everything we've done, ending the year with another beatdown at the hands of an instate rival, and then promoting from within after the big show of firing not just the coach but also the AD mid year. So Key likely needs to beat UGA to earn the job imo. I also doubt that not hiring Key will really upset the fanbase that much if it's coming off a 49-3 beatdown or something similar. Also, if we go 4-2 over the last 6 there is a very good chance that the two losses are UNC and UGA to end the year which just heightens that effect.
4. Potential problems going forward. If you hire Key and then we slip back to 3 or 4 wins next year, then what? You fired a coach and AD in a big spectacle mid season just to promote from within and then to the exact same thing? That makes the President look like a joke and I can't see that happening. A new staff will get a honeymoon phase that Key won't. That's why imo it'll take something really amazing, like beating UGA, for Key to get the job.