Robots.txt has failed as a system, if it hadn't we wouldn't have captchas or Cloudflare.
In the age of AI we need to better understand where copyright applies to it, and potentially need reform of copyright to align legislation with what the public wants. We need test cases.
The thing I somewhat struggle with is that after 20-30 years of calls for shorter copyright terms, lesser restrictions on content you access publicly, and what you can do with it, we are now in the situation where the arguments are quickly leaning the other way. "We" now want stricter copyright law when it comes to AI, but at the same time shorter copyright duration...
In many ways an ai.txt would be worse than doing nothing as it's a meaningless veneer that would be ignored, but pointed to as the answer.
Failing to solve every problem does not mean a solution is a failure.
From sunscreen to seatbelts, the world is full of great solutions that occasionally fail due to statistics and large numbers.
And if you feel like rolling out the "welcome friend!" doormat to a particular training data crawler, you are free to dedicate as detailed a robots.txt block as you like to its user agent header of choice. No new conventions needed, everything is already on place.