You are vastly overestimating people's willingness to deal with bullshit, when the product does not have a real lock in.

It would be incredibly easy to have a company offering their ChatGPT over WhatsApp or iMessage, and get people to start using it instead of an ad-ridden GPT app.

Maybe. But maybe you are vastly overestimating people's willingness to give a fuck, as long as they get what they came for. That is why ads rule.