That's actually a very interesting idea.

What if it changed the game so that instead of exams causing stress and discouraging learning, people were incentivized to learn at their own pace and eventually pass the exam?

This situation already exists with many professional credentials, such as CPA, CFA, FSA, etc. They tell you what will be on the exam, provide practice problems, lots of study materials are available from different sources, and you take it when you're ready.

There's also college exams that have a similar function (CLEP is the acronym I think?) but they're not really highlighted as a preferred path to students.