Hashcash was a really intriguing idea. But it fell completely apart once it was realized that spammers could use botnets to do all the proof of work for free.

Plus Hashcash isn't a great solution for mobile devices especially, as it uses up battery and low-end devices particularly suffer. And obviously challenges have to get harder every year to keep up with high-end hardware, which makes older hardware become increasingly unusable even faster for web browsing.

So unfortunately it is not a solved problem. That's why device attestation still seems like the least-bad solution right now.