[stereolambda]

There is a difference here without the society preference and what you should do as an individual. As you are saying, in crowded fields breakthroughs may be likely (or not, due to overexploitation, rampant fashion instead of principles thinking and people digging themselves too deep into one paradigm), but in any case, you are not likely to be participating in them if you're not already a strong player. Thus, if you are doing something like research or development of new things, your marginal impact will be probably tiny.

I would put two caveats on this. First, if you are globally in a good position (world's top university, good connections to get into places) you may be positioned to meaningfully get even into a mature field. Second, if you don't want to do much research and exploration, it's reasonable to reap benefits from specialist knowledge of already proven fields. Just keep an eye on other options to transition to if everything eventually crashes.

Even in crowded areas you should benefit from wide knowledge of horizons and fundamentals beyond what is now fashionable. It may mean just more ideas and perspective. I work in NLP with an interest in AI, and I've dived fairly deep into currently out-of-scope things like rules-based NLP, symbolic AI and biological neurons, their physics and simulations etc. I hope this gives me more viability that your average deep learning guy from a moderate background.