Great advice for a YC startup, but not how big companies that already have large user bases operate. Big companies have a metric they want to drive, then look for big opportunities, simply because dominating a “niche” is too small an opportunity to make a dent in a big-company metric like DAU, time spent, etc. If you do want to start with a niche, it can be super hard to justify continued investment from management, given that there are so many other bets that can drive larger near-term changes to metrics.