Isn't SRD 5.1 essentially just 5e? Unless they start releasing all the 5.5e/One stuff under the same license I can see them just not releasing that under a permissive license. Of course I could be wrong.
Sure, but the outrage from this change came about revoking the license for past works. By putting SRD 5.1 under CC, the situation for many indie creators of working with the SRD becomes clean and clear. Hasbro can do whatever they want going forward with D&D.
Game rules/mechanics usually can't be copyrighted other than just the literal text of descriptions of them. Does this give anything you couldn't have by cloning the rules with different wording flair?
The thing is, compatibility with D&D wasn't necessarily about mechanics. Fundamentally TTRPG mechanics are really just about simulating probability distributions with dice. There's the broader question of how to use a given probability distribution where.
What D&D editions do is develop a set of base mechanics, basic probability distributions, and then create a framework on how to apply them. This includes monsters, races, and classes which have particular attributes or play feel. This includes common roleplaying conflicts and guidance on how to adjudicate them. What a lot of indie RPGs that used OGL 1.0/a did is they made references to things from D&D and used them in ways inspired by D&D. Think "elves" or "faeries" being associated with the mechanics and tropes you'd expect. Now with the new CC-BY-SA 5.1 SRD, you can make explicit references to Elves in derivative works and also license it under CC-BY-SA.
There were always completely separate systems that borrowed nothing from the play feel/world of D&D. Those communities/creators had nothing to fear.
"can't be copyrighted" may be true in the ideal, but there's plenty of haziness around the edges. The particular coloring of each type of dragon is mechanically unimportant, but is a nice mnemonic for the type of breath weapon damage they do. Is dragon coloring function or flavor? If you're a small publisher, you don't have the lawyers to compete with Hasbro in court long enough to find out.
SRD 5.1 doesn't include character creation rules or feats, so it's not even enough from just the PHB/DMG to play. But it's most of what you need, and the what's left can be easily replaced.
The CC grant from the draft OGL 1.2 announcement was much less - it didn't include classes, spells, or creatures. Opening the entire SRD under CC BY makes cloning 5E trivial.
I genuinely think this is WotC giving up on 5.5/One being open. They're going to double down on the Dungeon Master's Guild and its IP-friendly, Hasbro-owned license as the way to create and distribute fan content.