>As I understand it, React was an attempt to shoehorn "immediate-mode UI"[1] on top of retained-mode UI

the problem is that the typical modern web page is considered as a combination of immediate-mode and retained-mode.

therefore, as it is wasteful to update all the page when only a few components on the page change, people want to only update parts of the page immediately when changes happen.

furthermore the typical modern web page only makes sense in the context of the modern web site, so some pages are almost everything on the page needs to update all the time, some pages are nothing here needs to update until the user does anything, some pages are nothing here needs to update at all, and other pages are just a couple components need to update all the time based on various things.

This context of what a web site is like is very much determined by the type of the site, where many e-commerce sites are affected by all sorts of data that occur outside of whatever page the user is on and need to update all sorts of different components on a page, many governmental sites are almost just static, with all sorts of variations in between.