[breuderink]

Could this training effect of a PhD perhaps be the reason that people switch fields? When starting, one's field of research seems very promising and exciting. At the end though, one knows enough to see limitations in the field. In addition, PhDs are trained to approach problems differently. I think this knowledge and a different mental attitude can make other fields seems more worthwhile.

[ser0]

At this point in time, I would say yes. The start of a PhD teaches you a lot of useful research skills. However, the academic/research environment do not necessarily put them to good use.

This can lead people, such as myself, to investigate the value of a PhD itself and find that the career prospects, financial rewards, and even intellectual rewards, are difficult to align/justify.

I think that a reasonable conclusion is indeed that switching fields would generally be a good move, unless you know yourself to be an outstanding researcher within your field of expertise.