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?
Rate limit individual clients.
Let's play this out: how do you determine individual clients? By ip? By seasionid?
How do you "determine" individual clients to show them CAPTCHAs? Yes, you can, and probably should, make some use of IP addresses, although that would work better if idiots hadn't polluted the Internet with quite so much NAT.
But you don't have to, and you definitely don't have to completely rely on it. Look for a cookie. If you don't see it, route the client through a page that sets it.
Yes, this is subject to flooding attacks... in exactly the same way that every CAPTCHA system is subject to flooding attacks. But it actually uses fewer resources per request than showing the CAPTCHA would.