99% of users having no idea what "export chat history" means?
Yet Google Chrome managed to make Safari/Edge irrelevant.
I suspect that Claude couldn't make an "import from ChatGPT" button because OpenAI would make it difficult, so they'd have to rely on user initiative and technical capability (exporting to JSON and importing from JSON is enough technical friction that the average user won't bother).
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.