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PoW applies the same amount of work to everyone. We want that in bitcoin. We're peers transacting with each other.
We don't want that for websites. The value of a website is subjective: crawlers run by big companies can and probably will pay more to access your website, especially if it's valuable enough that you're considering PoW as a defense, see this quote in the article:
because apparently what we have to offer is worth spending a ton of cycles to calculate the Anubis challenge.
You need to make the PoW challenge difficult enough that it's no longer worth it for them. Then you need to consider that they probably have to wait much less than legitimate users on low-end devices because they can afford more powerful hardware. So you ruin the experience for your legitimate users long before you make it impractical for the bots.
PoW is indiscriminate, and thus the wrong tool. You want to distinguish the traffic, not make the experience worse for everyone. You want to do stuff like this:
It's impossible to tell with certainty which of these are bots and which are real humans — but chances are, if it's asking for an old commit in a random old fork, it's probably not a real developer trying to do their work.
Expect to lose some functionality, at least when accessing our resources anonymously.
See also this HN comment.
Why isn't PoW an asymmetric battleground? Because users and bots both have to do the same work? The same would be true for using Bitcoin for anti-ddos but I'm still interested in this.
Am I misunderstanding how you are thinking about it?