Is your dream a PhD in machine learning, or to work on difficult machine learning problems? Many people (most, probably) come out of their PhD less happy than t

Is your dream a PhD in machine learning, or to work on difficult machine learning problems? Many people (most, probably) come out of their PhD less happy than they were before, maybe there is a different way you can pursue this interest? If you even have a chance of becoming a PhD candidate, I assume you already have some skills in the subject that you could apply at a corporate job, on an OSS project, etc. I understand the appeal of "pure" research, but there are other paths as well. I don't know much about the field so I can only guess, but maybe you can find some people here or elsewhere to talk about this who have the relevant experience?