Even if example.com is unsigned, the delegation from .com to example.com will still be signed (including an attestation that example.com is unsigned). So lack of DNSSEC adoption by users of the TLD wouldn't save them here.

Sure. But that was not the issue with .de, it has about the same level of DNSSEC adoption as .com

DENIC screwed up the TLD itself, and .com/.net are just as susceptible.