New tech has an inherent disadvantage vs legacy tech, because there's more built-up knowledge. If you choose React, you have better online resources (official docs, tutorials, answers to common pitfalls), more trust (it won't ship bugs or be abandoned), great third-party helper libraries, built-in IDE integration, and a large pool of employees with experience. If you choose some niche frontend framework, you have none of those.
Also, popular frameworks usually have better code, because they have years of bug-fixes from being tested on many production servers, and the API has been tailored from real-world experience.
In fact, I think the impact of AI generating better outputs for React is far less than that of the above. AI still works on novel programming languages and libraries, just at worse quality, whereas IDE integrations, helper libraries, online resources, etc. are useless (unless the novel language/library bridges to the popular one). And many people today still write code with zero AI, but nobody writes code without the internet.