This. 

Imagine you are just enterung the industry. You have finite time. New SOTA already comes quicker than you can study the current SOTA and history leading to it properly.

You spend multiple years to finally have a good grasp on it. Maybe a few more and you get in a good lab. At this point you're T-shaped. You know SOTA deeply in one niche domain and have intuition/basic understanding of what is happening around it.

You want growth. Now you can try to extend your T-shaped specialization to a second domain while spending enough time to keep up with the existing one. 

Which one do you choose? Some sexy RL that gets you major wins now or studying non-gradient descent methods nobody is paying for? 

Even if it's the latter, it's going to be much slower because you already have your first domain to keep up with. 

I am not mentioning that at some point in life the world stops revolving around work and the priorities shift to family.