If anyone was putting up real money I'd be happy to rank a new website for just about any keyword you can imagine I also think the Gold in the Google mine will never run out.
Companies like that are not exactly a "niche" - they are kingdoms. Specializing in powdered doughnuts would be a "niche". The word "niche" often implies "limited" resources. In a way, a "niche" would be like going back and re-visiting creeks to pan for gold after floods, but after other people had control of the main mines. Yes anybody can start a niche, just like anybody can pan for remnants of gold. Also, once ANY niche gets started, it can be shared by others.
Things are tough online, only the innovative will come out on top. If a niche isn't working out, you may need to change your approach, or even target a different niche.
I'm thinking more in the way of the premium coffee [$2 cup of coffee] niche market. Premium coffee started out as a niche and has become more mass market, dominated by Dunkin' Donuts and Starbucks Yes indeed it is so true. Niche markets develop into mass markets and it becomes squeeze the small guy.