Yeah, I had the same question myself. I think that's what you would want to do to make it airtight (plus some amount of rate limiting or flagging for devices that are part of dedicated device farms).
But even if not, there's still value in raising the barrier to entry. For example, you can buy 1000 reCaptcha solves for $1-2 from various captcha-solver services. And yet that $0.001-per-request fee does discourage mass-scale bot attacks.
99% of the crap and fraud comes from ads, aka Google. Thanks, Google. Just run an ad blocker, there goes most of the scams you'll see.
Also putting QR codes before every webpage doesn't make the web less shitty. It obviously makes it more shitty. And this will 100% be used for fraud. Phishing websites can get away with QR codes now, great.
How though? Can you also avoid DDoS simply by designing your system to not care if the requester is a bot or not.
Let's say I'm running https://grep.app/ for example. AI bots start heavily using it, costing me a ton of money. How would you magically design this so it doesn't matter if the end bots are using it?
But even if not, there's still value in raising the barrier to entry. For example, you can buy 1000 reCaptcha solves for $1-2 from various captcha-solver services. And yet that $0.001-per-request fee does discourage mass-scale bot attacks.