Often things get worse before they get better.... were I the CEO of Mozilla this is the vision for the browser that I would have. Electrolisys is one example... something that needs to happen for the browser to advance but also negatively affects the rendering engine. But that is ok because the rendering engine is on it's last leg as is the extension model.
Future firefox will be a blank slate, with a UI rendered almost if not entirely in HTML and portable webassembly. That means that the chrome extension model is MORE powerful than even on chrome as unlike chrome Firefox is most likely going to use an HTML based UI rather than a native one. The evidence for this is pretty clear... if they aren't going to use XUL and XPCOM what would be the point in replacing it with more of the same?
We'd almost certainly have extensions written in Lua, ruby and who knows what else... people that wanted thier extensions to be robust could write them in rust the same language as the engine itself.... and it would get optimised down from webassembly into native code. To me that sounds like like ti would have WIN-WIN for all developers and users of mozilla tech.