A niche has a coziness that is very appealing. Keep your eyes straight ahead. Don't deviate from the path. Maintain your focus. Marketing experts often write of finding your niche and establishing your beachhead before you even think of moving further into the wider plains, if at all.
I won't dispute their wisdom.
The difficulty is that a niche can require a certain level of broader knowledge. Few specialties are immune from forces that come from far outside of their boundaries. As a management consultant, I've obtained extremely valuable information by studying history, government, philosophy, economics, drama, and psychology, just to name a few seemingly "outside" areas. That broader knowledge provides perspective while feeding a healthy skepticism about the current management fads.
As with so much of life, the choice can be "both/and" rather than simply "either/or." Knowing what else is out there can improve one's focus.