In engineering, there are always hot topics and non-hot topics. Whenever there is a hot topic, it's like everyone is working on it. No one wants to be left behind. More precisely, everyone seems to want to get some money out of it. Hot topics come and go, leaving behind lots of people with knowledge and skills that are not valued anymore.
When I first started my PhD, I thought I was working on a relatively "interesting" area. But as I am close to graduation, everybody seems to despise this area. They already moved on to the current hot topics like big data. Everywhere you hear people say: "Oh, xx area is not worthwhile studying anymore." When you submit a paper, reviewers tell you that they are not interested in research in this old area anymore. When you submit a propose, funding agencies tell you that they definitely don't give funding to this dying area. When you try to find a job, companies emphasize that they are looking for people working on this and this hot topics. No one values your experience in the old non-hot area.
Now I see why mathematician is the best job in the world. No one tells a mathematician what he or she ought to work on. No one tells the mathematicians to work on some hot topics. Because saying this, it's like telling the mathematicians that maybe you are not too stupid to work on this famous problem, this long-open problem; it's an insult to the mathematician's intelligence.
