Not really a question, but still and interesting conversation that started with:
The first part is a no-brainer: There are some people who simply are not cut out for long-distance hikes, physically or mentally. A 700 km. walk, or even the minimum requirement of 100 km., would be detrimental to their health. They would quickly fall ill or hurt themselves if they attempted it.
To read the rest of the posts, and the many comments that this received, have a look at the conversation on The Camino de Santiago is not for everyone in the forum.