That is such a great question and there is no easy answer. There have been enormous efforts to do better for at least the last 20 years. An entire organization, M3AAWG, was founded for that reason and it meets three times a year, bringing together all the people that matter for making the situation better. It's a great organization and the people are all really smart and awesome. The IETF is no slouch either, coming up with excellent new standards and improving existing ones, such as the recent update to DKIM.

That's about as good of an answer as I can provide: keep sending smart people to the conferences!

Signed senders?

It's simple, there's a standard, a new one, which takes into account SPF, DKIM, DMARC, ARC, and even DANE along with upcoming and purposed SPKF, DKIM+, DMARC2, and ARCv4. It should fix just about everything.

Hashcash, or BTC.