I see what you're saying, and I think I agree with most of it. However, I do take issue when people who get their email on such an "opaque black box full of heuristics and uncertainty" claim it's the sender's problem when email is accepted by the black box but not received by the black box's users.

If your mail server's admins are going to have your mail server perform complex operations that drop received email into something other than your inbox, however necessary and unavoidable those operations are on the modern internet, and you miss email you wanted to read as a result, that is something you need to take up with your mail server's admins, not the sender. If your mail server's admins are unresponsive, possibly because you're not actually paying them but getting email as a free service, that is still your problem, and still nothing to do with the sender.

I do feel a clearer understanding of the line of responsibility here would improve email for everyone.

The only real solutions I see to this are whitelists, blacklists, and Hashcash. The last has been ruthlessly stopped by large corporations in favour of reputation, because these large corporations all want to send spam. Personally, I believe accidentally missing an e-mail to be unacceptable. These same large corporations like to have their servers lie about accepting e-mails, because they want to destroy what they can't control.

It's because anti-spam isn't working against a blind natural phenomenon; instead, anti-spam is engaged in an iterated game against human driven spam.

Let's not forget that proof-of-work is bad for the environment, that message brought to us by the same corporations that callously waste resources and produce mountains of plastic.

I disagree, for two reasons. First, charging for email in general is not going to stop spam, although it will change what sort of spam you get. This includes Hashcash, especially now that you can rent compute capacity as you need it (so people who want to send out a marketing email campaign can literally pay for the Hashcash costs, were they exist). Second, Hashcash harshly penalizes legitimate senders of significant amounts of email, including mailing lists, who see their compute needs and thus costs go up drastically. Hashcash is a non-starter in anything like even a traditional, pre-spam Internet email environment, much less today's non-spam email environment.

(Plus, active criminal spammers have plenty of compute capacity they can rent for cheap, cf botnets for hire.)

I disagree, for more than two reasons. As e-mail isn't time-sensitive, it would be feasible to have proof-of-work requirements that last into the tens of minutes or hours. This basically kills the drive-by spam. In a system without an authority, the idea of legitimacy loses most of its meaning. Regardless, Hashcash could be used as a way to get a sender address into a whitelist, and no address in a whitelist would need Hashcash to be accepted. Hashcash could be used entirely as a signal for automatic categorization like this. Free Software mailing lists, and similar such lists, could be collected into some whitelist for interested parties.

Hashcash is a non-starter in anything like even a traditional, pre-spam Internet email environment, much less today's non-spam email environment.

Explain why. I'm not seeing it. The main opposition to Hashcash are the aforementioned companies which want to send their spam to everyone.