I mean, the same thing that is happening right now, right? The point would be that websites would not be built to only allow trusted clients-- it would still have to allow all clients. If they wanted to remove this 10% thing, it's not like the entire web would instantly stop being built to allow untrusted clients.
I'm not sure this is true (keep in mind: weak grasp). This 10% would push websites from specifically blocking untrusted clients-- but if they got rid of the 5%, it would not magically change all the websites to block untrusted clients. They'd still need to update to do this.
I don't want to come off like I'm defending this though-- I really just don't know enough to say.