If one request to the site generates more revenue than it costs in resources, the bot problem is solved.
The author says that he is getting 15 bot requests to his site per second. That is about 36 million requests per month. How much does it cost to serve those? $1000 would seem high.
$1000/36M = $0.00003 per request.
How long would a crypto currency, that is suitable for mining in the browser, need to be mined before $0.00003 is generated?
If it turnes out it is a few seconds or so, the solution would be nicely user friendly. A few seconds of CPU time for access to the site. No ads needed to finance the site.
It is kind of telling, that Bitcoin started as a spam blocker. The original "hashcash" use case was to use proof of work to prevent email spam.