By and large, when you talk about firing someone, regardless of his profession, it is done because of one of two things, or a combination of the two.
First, you can fire someone because of past failings. A salesman may be fired because he has failed to achieve a particular quota set forth. Donnan was fired for his inability to beat any of Georgia's rivals with any real consistency. Damon Evans was fired for basically disgracing himself and the university.
Second, you can fire someone due to a lack of confidence in future success. While this obviously feeds off of the past failings idea, it is still a choice made independently of that in many cases. You may choose to keep a salesman who's come up short of his sales goals, because you know that he has talent and ability, his numbers have improved, and you know that this is a struggling economy. Or, you may fire him because you have not seen any noticeable improvement, and you simply have no confidence in his ability to get on track.
When you hear Athletic Director Greg McGarrity talk about evaluating everything on a week-to-week basis, he's taking both scenarios into account. It's a balancing act, and one to which a great deal of thought must be given. Certainly, the fact that Georgia has had great success under Richt shows that great seasons are a possibility. Yet, the last 2+ years show that continuing to lose is also a conceivable eventuality.
Many fans want to see a number of victories or an SEC East/SEC Championship labeled as the low-end threshold for keeping Richt. Putting aside the ridiculous amount of scenarios that would create a problem with this, it's still more about the confidence McGarrity ends up having in future performance.
Everybody is past having to accept that we saw some "good signs" in a loss. What must be decided is this: Are those good signs going to lead to actual wins, or is Georgia destined to continue to show good signs while mired in ~.500 seasons under Richt?
It would take a complete debacle by Georgia for this decision to be made mid-season. So, barring that, fans are just going to have to play along. While you may have made your choice already, McGarrity has not.