I mean.. I think you’re answering your own question here.
You can argue that the web shouldn’t be open. In fact, there are many arguments for that, which I don’t mind arguing against.
There are many things that do not belong on the web, precisely because it’s open. For instance, a registry of people’s political views. Or naked pictures you don’t want the world to retain forever. And so on. The fact that the (open) web is not suitable for everything has been true since its inception. Openness comes with trade offs.
The honest way of putting it, is that WEI wants to make the web less open so that it can have more content, or protect content better.
On an opt-in basis, this is fine in theory. But WEI would never ever be opt-in with meaningful consent. It’s entirely dead in the water there, because non techies will not understand what or why this is “needed”. Heck, people don’t even grok cookies. In practice, this will be enabled by default, which is exactly the fear. Alt browsers would be bullied to support it, and users would be forced to use it.