[crazygringo]

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.

[walletdrainer]

Why would we stick to hashcash when we have decades of work built on it?

You can trivially substitute device attestation with e.g. proof of burn.

Instead of tying attestation to specific hardware manufacturers and configurations, it can be trivially tied to monetary value.