I think ads will take the form of insidious but convincing product placement invisibly woven into model outputs. This will both prevent any blocking of ad content, and also be much more effective: after all, we allude to companies and products all the time in regular human conversation, and the best form of marketing is organic word-of-mouth.

You have no guarantee the API models won’t be tampered with to serve ads. I suspect ads (particularly on those models) will eventually be “native”: the models themselves will be subtly biased to promote advertisers’ interests, in a way that might be hard to distinguish from a genuinely helpful reply.