Seems to me that if something is smaller, it WILL fit!
The question becomes:
"How well does it fit?"
"Does it move around or is it tight enough?"
"Does that little extra thickness in the back prevent it from fitting?"
Seems to me that the answer to all of these will depend on the material. Hard plastic cases are probably a lost cause. Things with give, like some rubber and perhaps leather cases, might still work.
There's no way to know until people try, but I'll bet anything that SOME old iPhone cases will continue to work "well enough."