For argument's sake, I'd suggest a "good" idea that's too far afield from your niche simply won't be good. Your audience won't like it, and it won't land.
I've certainly published articles that don't fit my niche because I liked the idea. When I did, the articles inevitably failed.
Worth noting, however, is that the bigger your audience, the more you can expand your niche. Tim is a good example of this. As he's gotten more readers, he's expanded the things he writes about.
That's actually another point I originally intended to include in my article, but it got too cumbersome, so I cut it. The short version is that new writers should niche down pretty hard. However, as they gain audience, they can (very slowly) expand the topics they cover without alienating readers. I mention this because I felt like Tim was writing more from his perspective now than from when he first started out. Sure, once you have 300,000 readers, you've earned the ability to broaden range of topics. But don't tell that to the writer with 300 readers. That writer needs to focus.