Maybe they should also work on what their users are sending as well.

Unfortunately, the people abusing these are very persistent and adding more checks will just increase the sophistication of such attacks which works against the interests of the target.

This stops disgusting yourself as a legitimate service.

Charge for the service. How much spam do you get from Fastmail?

If they did that you’d decry censorship.

Allowing basically ~whatever, but proving the sender address is an obviously good approach to me.

I don't know what "v=DMARC1; p=none;" is proving to anyone when DKIM and SPF are already aligned but sure whatever. Just wish they'd given some warning!

email is weird, without this it doesn't perform the checks at all

Agreed. There's no additional verity provided by adding a DNS record with a static line of text. Reminds me of airport security theater: meaningless hoops for citizens to jump thru under the guise of stopping bad actors.