I asked earlier what from the 3e SRD is needed but got no answer, so nothing I can add at this point...
I didn't read this thread until today or I would have answered sooner. And I can answer not just with some invented hypothetical, but with something that I had actually planned to do
that I cannot do without access to the earlier SRDs.
Maxperson hit on this a few posts up, but the biggest thing you're missing, and that Sacrosanct was missing in all their posts where they went on about just rebuilding the 3.Xe rules like the OSR games did with earlier editions, is that the 3.0 SRD is not just the core rules. There's a heck of a lot of content included in the earlier SRDs that isn't in the SRD 5. Just look at the monsters alone: there were about twice as many monsters in the 3.5 SRD than there are in the SRD 5. And that's not
counting the monsters from the Psionic SRD, and the Epic Level SRD, and the d20 Modern SRD... There are a heck of a lot of monsters that were available through the OGL 1.0 that won't be available if earlier SRDs are no longer usable. (And spells, and magic items, and...)
Now, as for my specific example of how this affects me... one project I'd been working on is sort of a 5E-compatible version of d20 Modern. (Yes, I know such games already exist, but there were things I thought made mine sufficiently different. Also, honestly, given how many projects I've started, it's entirely possible that I never would have actually gotten around to finishing it, but that's not pertinent to its use as an example of something that requires the earlier SRDs.) Now, the rules
I didn't need the d20 Modern SRD for. They were intentionally much closer to the 5E rules than the original d20 Modern rules. I was having classes corresponding to attributes (sort of), as a callback to how d20 Modern did that, but I was doing it in a different way than d20 Modern did; I wasn't just using the d20 Modern classes, and just the concept of having classes matched to attributes isn't sufficiently distinctive I'd be likely to get in trouble for it. So fine, for the rules
the OGL 1.2 would be fine. In fact, since I'm not using any of the specific D&D classes and monsters and whatnot, I could probably get away with just using CC-BY, though I haven't looked closely enough at exactly what they're releasing through CC-BY to be 100% sure of that.
But I wanted to use the Urban Arcana setting from d20 Modern. And there's a lot
of stuff about the Urban Arcana setting that's in the d20 Modern SRD. I couldn't use the name
"Urban Arcana", but they had pages and pages of information about NPCs and organizations that were all Open Content. I can't use that without access to the d20 Modern SRD. The rules, sure, maybe, but not this particular content.
So you can just make your own content instead
. Well, of course I can. And I was
going to make my new content in addition; the (renamed) Urban Arcana setting was just going to be one of several settings released for the game, the rest of which were going to be original. But you know, it's fun
sometimes to play with other people's toys, and to build on something from the past. I wanted to update the Urban Arcana setting not because I couldn't make up my own material, but because I thought it would be enjoyable to have something done with this old material, and I had ideas for some interesting directions to take it.
So yes, there are reasons people might want access to earlier SRDs. As for my game, if I ever get around to finishing it... eh, I guess at this point if WotC doesn't end up releasing the d20 Modern content through the new OGL, I can still release the game, just not under the OGL; like I said, I'm really only using the base mechanics from 5E so I don't really need the OGL for that, since they've been released under the CC-BY, and I can release my game under the ORC. There were some monsters from the 3E SRD and the d20 Modern SRD I'd been planning on using, but I can live without them; it'll just be disappointing to not be able to use them. And very disappointing to not be able to use all the Urban Arcana stuff I'd had plans for. If WotC does
decide to release the d20 Modern SRD material under the new OGL... then honestly I'll still
keep anything I'd need the SRD for out of the core books, and if I do use the OGL at all I'll only use it for the Urban Arcana setting, and confine anything I want to use from the 3E SRD or d20 Modern SRD to there. But anyway, I hope this shows that there are
reasons to want access to the pre-5E SRDs, and people who want them to be included aren't just complaining for the sake of complaining.
(Fortunately, I already hadn't been planning on using any third-party OGL material, or I'd be in real trouble... if WotC's "deauthorization" of the OGL 1.0(a) does end up causing issues with use of 3pp Open Game Content, Porphyra RPG
is really up a creek...)