As an outsider: what exactly is it about "routers" that justifies so much change? Aren't there plenty of established designs to copy from other stateful UI frameworks?

I think supporting React Server Components is at the core of this issue. A ton of changes were necessary to support RSC and a different router is just one of many things that changed. Important features have been sacrificed to make it happen. You for instance no longer have access to the request/ response object in middleware or layouts. Currently playing with Remix as an alternative.

We are using Remix at work. We are super happy with it. None of the nonsense artificial limitations of Next, great integration with express, and a really simple model which, if you ever used a traditional “MVC” framework, will make you feel right at home.