They spent a huge amount of complexity supporting mailing lists that claim mutated messages came FROM the original sender instead of FROM mailinglist@example.com. Is that use-case worth the additional effort?
Yes! Forwarding is the reason mail flow authentication does not work very well today. Fix the forwarding use cases (and by extension: mailing lists & out of office arrangements) and we can finally just tell all senders:
"Your domains do not match, please use DMARC so we can automatically detect whether that is OK. No excuses, no exemptions, your use case can be dealt with using DMARC!"