OCR is a more expensive option, right? I don't think these fonts need to stop ai from being trained; I think they just need to make it more expensive and difficult.
Also more expensive doesn't mean that something won't happen, only the dynamics of how it happens. For example if you put all your documents in images then some service might just sell the AI providers the text. That service may do underhanded things like bundle OCR in an app that does something else and use your phone to get the text out of these images all day.