> In what sense am I underestimating it?

Given the design complexity you're aware of, the need for practical testing to iron out the kinks, and the burden of converting anything important to a new GUI toolkit, I find the chances of this happening rather slim.

On one hand, AI tools can help develop the toolkit faster and easier. On the other hand, the same tools just make the task of dealing with GTK and Qt easy enough for many not to bother, while enjoying the network effects of relying on a large and established project.