I expect to see publishers (both 3PPs using the WotC 5.1 SRD and indies producing their own games) to create their own SRDs to release under CC BY 4.0. This gives downstream publishers the option to keep their content closed or CC, whereas an SA licence would require share-alike.
I'm still not clear on some elements.
If I use CC BY 4.0 content in a product, can I then release that product under a different CC licence such as CC BY-SA or CC BY-NC?
What exactly does "indicate if they made modifications to the licensed material" mean? The example given is, "This section is an excerpt of the original." Does one have to do that everywhere there is a modification? That would probably double the word count of most clones, and make them unusable in practice. Or is it sufficient to say, "Chapters 3-6, 8, and 12 use portions of the original text."?