I am glad that the FSF post explicitly mentioned the source requirements of GPLv3 when ones are considering adapting CC BY-SA 4.0 materials into GPLv3 works. I fully support this precaution, but this also makes clear the burden when one is to make use of others' CC BY-SA 4.0 blobs for GPLv3 works. How to get and verify s/he has the right source materials from which the blobs can be faithfully regenerated? What happens when the source materials are not explicitly BY-SA 4.0 licensed?

Other the other hand, now that it is officially one-way compatible, perhaps there will be a new norm for people to always release CC BY-SA works with the source materials. :-)