From the answers to this question. We can clearly know that novelty is a basic requirement for a PhD degree.
Now, consider the following statements:
1) Is Goldbach conjecture true?
2) Is P = NP?
All the questions have a lot of attention in that particular field and are not easy to get answers for them. But we cannot say it is impossible to get answers.
Suppose I'm a student and my PhD thesis should be novel. I am highly interested in solving one such question.
In this case, either my supervisor will forbid me in doing research on such question or there are more chances that I may not get PhD because of the sole reason of novelty.
Hence, to the best of my knowledge, either directly or indirectly, the PhD is narrowing down to the topics of recent origin, relatively easily publishable, less saturated etc.,
Isn't it true that the research question has been influenced by novelty aspect of PhD?