As far as I can tell, there is only one way for humans to grow:
Challenge yourself and then rest.
I know that it sounds overly simplistic, but failing to follow this is literally the cause of every plateau I have ever had and every long term failure.
If you haven't experienced it, I'm sure you've seen it:
The Actor wins an Oscar and then disappears.
The Writer has success with their breakout novel, and never matches it again.
The athlete becomes a champion, and then a train wreck.
Maybe you've seen it in smaller ways. You get to a point where you are comfortable in your job and then never get promoted again. I play rhythm guitar well enough to accompany myself, but that's it (and I've been playing for 15 years)
The reason for this is that we cease to challenge ourselves or we fail to give ourselves the fuel and time to recover.
there are hundreds of reasons for this:
You have a family to support, you can't go and take a bunch of risks.
You don't have the time to work out every day; or if you do you don't get enough sleep to maximize it.
You are an established young adult novelist now, stick to the formula.
But I think that this is a mistake.
If we aren't reaching for bigger challenges, we don't just stagnate; we regress. Resting on your laurels is more likely to come from a desire for stability than outright laziness, but either way you are "killing your gains".
The other side of this is working yourself into the ground. Maybe you've never quite hit your goal, so you double down and work harder and harder, never resting. In fitness, we know that such a method would break down the very tissues of your body, but it's true elsewhere too. If you are a writer, and you never take a day to just think; you won't learn from your mistakes. You will again become locked into a routine. You will fail to experiment.
In short, you will have all of the disadvantages of someone who got comfortable; but without the comfort.
The real difficulty is knowing what is lacking. Are you pushing too hard without sleep? are you half assing it? The only way to know is to experiment. change the workout, write a novel if you are a screenwriter, do a drama if you are a comedian, shake it up. sleep in for once. Take a vitamin. Whatever.
People do not do well with standing still.