I do grant that WotC never came out and explicitly said, in so many words . . .
"We're trying to patch up pubic relations after our missteps around the OGL. And, of course, we want to do that at minimal expense and effort (that's why we went with CC-BY so suddenly, after all, it was faster and cheaper than revising the OGL again). And we see some people are still worried we might, years from now, revive the de-authorization theory to go after OGL-licensed games derived from our released-in-2003 SRDs. So we're just going to assign someone, as their fifth priority or something, to check those older SRDs for anything that we wouldn't want to release under CC-BY, and when that eventually is done, then we'll license them that way.
"Of course we're not going through the 4th edition rules and producing a usable SRD for that edition. That would be much more expense and effort than reviewing the existing OGL-licensed SRDs, and it wouldn't be addressing the concerns of outsiders that we might go after existing games. That goes as well for the TSR versions of D&D, which would be even harder to make SRDs for, and since they generally have serviceable 'Open' retroclones already, based on the released-in-2003 SRDs that we're already going to CC-BY."
. . . but that's what I always understood them as saying. I'd count it lucky if pages 4-84 of the 2009 "4th Edition System Reference Document" actually got released under CC-BY.
As far as a timeline, I expect we'll see the 2003 SRDs released under CC-BY around the same time as the SRD for D&D 2024, simply because dealing with the D&D 2024 SRD naturally brings up the topic of the promised older editions review, and it'll be easy for a manager to assign the job of doing the IP reviews to a minion as a single bundle.