TL;DR: "Kill your babies"
No, not in the literal sense. The idiom "kill your babies" has many different variations, the most common one being "learn to let things go."
I first heard the phrase "kill your babies" over 20 years ago when a prominent director was giving me notes on a script. There was a scene that he didn't fully understand, and I tried justify why it was in there. He wasn't buying it, and casually looked up, muttering, "sometimes you have to kill your babies."
Ultimately, he was right. No matter how infatuated you are about an idea, no matter how much you think it's right, sometimes you have to learn to let things go.
Carte blanche is a rarity once you start working with networks, studios, or even with other crews. Even those at the top—Abrams, Fincher, Nolan, Spielberg, Scorsese, and Tarantino—are not immune.
Filmmaking is naturally a collaborative process, so you're going to run into different personalities and contrasting ideas. Some you may not like at all. When you cross that path, it's important to be constructive. Figure out what best serves the stories. Plus, try not to burn any bridges in the process.
Though the definition of Hollywood has undoubtedly changed, it's still small. The last thing you want to do before getting a foot in the door is to have a bad reputation.
Now, I'm not suggesting to be complacent. To push something aside that you truly believe in. When you're clear, concise, and justify why you think something needs to happen instead of wanting it to happen, it's a step in the right direction. People will listen. But when an idea isn't working, or it's stagnant, or doesn't meaningfully move the story forward, it's time to kill it. Let it go and, one day, it may find its way back in another form.
That said, you will still have to learn to pick your battles. You read about it often near a film's release that a director had to cut out several scenes in a movie for one reason or another. Or that a writer had a completely different idea for the story.
In episodic television, network executives tend to give notes on every episode, and you will have to find a way to incorporate them. In a way, they're creative compromises, that by the end, hopefully tell a better story.
Have you ever had to let an idea go? Have other advice? Share it in the comments below.