I want to get a general idea on how in-demand various CS subfields are (going to be in 6 years), so that I pursue my phd research in an employable area.
How do I go about doing that?
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When I was in school, the thought of a man-portable device that can communicate with any other human on the planet was in the same league as plasma beam weapons, space dreadnoughts, mind-reading aliens and "The Force". 25 years later and it's a 100$ device kids get for Christmas.
Moral of the story? Don't try to predict the future. It's not worth the time. It will be different anyway. Do what you like best now and adapt to whatever happens.
A PhD is fundamentally a research degree; it is a piece of paper that says "this person is qualified to do research". While the highly specialized domain knowledge you'll acquire in the process of doing that research would also be valuable, it's entirely possible to get that domain knowledge much more rapidly through things like gaining experience in industry or a Master's Degree that focuses less on original research and more on doing coursework that focuses on mastering the current state-of-the-art rather than finding an opening to expand that state-of-the-art.
A PhD would, however, be quite valuable if you want to go into academia, or a research-focused role in industry like the R&D department of a large corporation.