"Getting a PhD means that you are the world's leading expert in your field"
No, it doesn't.
Lots of people have PhDs. If they are each the world's leading experts in their fields, then each is the leading expert in a field of one.
I have never heard of a PhD examiner saying, "it's good work, but I don't think you're the best in the world, so sorry but we have to fail you."
It's ridiculous, if you think about it for more than a few seconds.
Getting a PhD is the beginning of your career, not the pinnacle. If you carry on in academia, that's when you can develop your skills and reputation and body of work. Maybe you will become the world's leading expert, but it takes a long time.
PhD students are under enough pressure. These kinds of ridiculous-but-pervasive ideas really don't help.