Presumably, they still want the free tier to help acquire new customers?

You could say they should separate the human users from the slopsters in any tier. This could be done with some quota/rate limiting model.

But, if they are either on the fence or believing that agent-based development is the way forward, then that new reality is the one they would need to support. Cutting it off will send those customers elsewhere.

And, if you've managed to handle the new workload, it is basically in the noise to also handle artisanal human developers in the background on the same solution...