SSGI is not meant to replace DFAO, they are completely different things and can be used alone or together.
SSGI is in screen-space so it can only give you small-scale detail (you can crank up the scale but you’ll get more off-screen artifacts), while DFAO is in world-space which means you get occlusion from objects that are off-screen.
SSGI rebounds light and works on any light, while DFAO occludes light and only works on ambient (sky) light.
Working with only GI you have to think of lighting as “additive”, i.e. your sky light should be the minimum desired ambient light (start with a dark base) because light rebounds will add up light to your scene.
On the other hand AO is more of a “subtractive” way of lighting, i.e. your sky light should be the maximum desired ambient light (start with a bright base) because AO will make things darker on occluded areas.
The fact that they work so differently makes them a good cause to use them together: use an average desired ambient light, knowing that SSGI will brighten it up in some places (on a smaller scale) and DFAO will darken it in other places (on a medium scale)
Couple them up with a larger-scale AO or GI solution (such as LPV) and you’d be covering small, medium and large scale real-time AO and/or GI, which is exactly what Lumen in UE5 will do (except Lumen will supposedly blend between the different-scale solutions instead of just stacking them up)
_
btw the infamous Cube problem with DFAO is due to non-uniformly scaling (and excessive scaling in general) of the asset. I thought by now it was widely known that scaling + DFAO was a limitation 