I always think 'stick to what you know' is more of a put down than sage advice. It's just a nicer way of saying 'don't give up the day job' - an implication that whatever you're trying to do is out of your reach, beyond your ability.
In some contexts that's a necessary recognition. You don't want an amateur with no flying experience landing your plane, or to be operated on by somebody who's just watched a lot of House. But come on, how often are those examples actually based in reality? It's the smaller risks - the ones we think will expose us - that we're really worrying about when we think about not sticking to what we know.
I've written before about the fear of failure, and how it stopped me from even trying so many things. It's easy to decide after one or two bad experiences that something isn't 'for me', to bow out before I can be further embarrassed.
I wonder how many times we all tried to walk before we managed it? How many times our parents had to guide us to the potty before we learned the signs to make it without help? How many times you fell off your bike before you could confidently sail down the road without your stabilisers?
Failure is the mainstay of the learning process. If we don't take the chance to fail, we don't know what to correct to improve. If scientists don't get negative results on tests, they can't narrow down the field to find what they're looking for. It's not actually normal to get things right first time, or even third or fourth.
So why do we have to stick to what we know? At what point is it decided that we are 'finished' and there's nothing more to learn, no more summits to reach, no more skills to master?
Come back to what you know, by all means - but don't stick to it. Expand your horizons; learn new things; plan fresh adventures. Stretch yourself to new views of the world. Don't let the fear of failure stop you from even trying, and trying again and again.
One of my mantras is that failure is being successful at the things that don't matter. It follows that I need to pursue the things that do matter, and they're not all going to be within my existing sphere of competence. I'm going to have to risk failure, embarrassment, falling short. I'm going to have to ask for help.
So if you see me backing out, backing down from what matters - challenge me. I need not to stick to what I know. I need to grow.