What you suggest is explicitly not a purpose of robots.txt per RFC9309[1]:
"These rules are not a form of access authorization."
HTTP 429 and HTTP 403 are what servers are meant to return to clients to slow them down or tell them to stop doing something without having first gained authorisation.
I wouldn't want them to. The whole point of using agents to do stuff on the web for me, is for them to do the stuff on the web for me.
This is the reverse of "do not track" case. It'll not be effective because every service will set it to DISALLOW by default anyway, because it costs them nothing, and for most services, it actually is what they want anyway - most of businesses on the web are making money on wasting people's time, and for that, they need to force themselves on people; end-user automation defeats that, so they actively fight it (and complain a lot).
e.g. a prompt of "fetch <article URL> and summarise it for me" is very close to what a human would be doing with a web browser, and doesn't seem to involve any kind of scaling issue.
That's the scale argument.
Same with browsing HN, btw. I have a row of 9 HN tabs open, all of them opened at the same time, as I scrolled the front page and middle-clicked on thread link to anything interesting.
The problem is that it is hard to distinguish your one off (which seems perfectly fine) from the tidal wave of bad actors.
Also let's not forget that innocent sites suffering from floods of scrapers are actually the minority here - this is just a special case; the main reason for the tension is simply that most websites and businesses on-line rely on users wasting their time, and cannot abide any form of end-user automation. Their business plans hinge on their ability to force themselves on you.