The old I-85 alignment (GA 13) will be able to relieve some of the traffic but nowhere near enough.
That assumes it even remains open. Based on what I've seen, I wouldn't be shocked if at least the southbound lanes of GA 13 are shut down for at least a couple days as well.
The loss of this segment of I-85 essentially severs the link between downtown and the north and northeast suburbs. I-75 toward Cobb County is going to be so overloaded as to be effectively unusable. Anywhere north of I-20 will be a nightmare for probably the next two months. Today I have a new reason to be thankful that I can take MARTA to work.
Through the weekend things will be a mess. Lots of folks will undoubtedly be taking an unplanned day off work tomorrow.
Beyond that, the saving grace here is that the southbound structure did not collapse. They'll need to inspect it to confirm but if it's in usable condition it can fit three lanes each way with no shoulders. So we'll probably see two way traffic on that side until the collapsed section can be replaced. Still a huge traffic headache, but much better than nothing.
The downside is that there's really no way to preserve northbound access to GA 400 with this setup, so that's shot until the bridge is replaced. Traffic will need to use GA 13 to get there.
Actually, that brings up an idea for what might be an interesting alternative: keep 4 or 5 lanes of SB I-85 on that structure, with only 1 or 2 lanes of NB I-85 on it. These lanes would have an HOV restriction during peak hours. Then, run GA 13 in contraflow mode - northbound traffic on both roadways. At the north end, traffic on the normally northbound roadway would be forced onto the ramp to I-85 north, while traffic on the normally southbound roadway would get shunted over to the northbound roadway here, and given the option to either continue up GA 13 or make the left to get to GA 400. With no through southbound traffic on GA 13, this left turn would be protected for the entire green phase.
In this setup you'd probably also want to close the offramp from GA 13 to Monroe Dr, and then restripe the NB GA 13 roadway so it narrows down to one lane prior to the ramp from I-85 NB. This way said ramp can add the second lane back and not need to merge.
Whatever the case with the roads, MARTA should run as many trains to North Springs and Doraville as they can. Though I don't know how much spare rolling stock they have (I assume the tracks themselves can handle more frequent service than is currently scheduled).