"MBTI isn't predictive"
The claim that mbti isn't predictive basically boils down to "mbti doesn't predict what job you're going to get", which is based on the fallacious assumption that people generally end up in jobs they're suited for. That's not how the real world works. Most people end up with whatever job they can get, not what they're suited for. Sometimes someone interested in ideas might become an academic, or someone who is detail-oriented might become an accountant, but it's a rare class of people who have the luxury of pursuing occupations that resonate with their unique set of talents and desires.
More generally, the idea that society is organized in a way that actualizes the aptitudes and preferences of individuals in measurable ways simply isn't the reality of the world we live in. There are personality tests that are predictive of real-world outcomes, but these tests necessarily and implicitly model the constraints that society places on individuals. They have to in order to be predictive, because these constraints are real. MBTI captures the true, often latent potential of individuals, not how their personality manifests behaviorally given the constraints placed on it by the world.
For many types, there is no straightforward path for actualizing their latent potential in our society. There's simply no place for them in this world; or the roles suited for them are so limited that most will never occupy them. The fact that MBTI doesn't predict what these people actually do in the world makes it, in fact, a more accurate personality assessment than any test that predicts real-world outcomes.