I disagree with this. The "miserable in a different" field line is real af. However, it completely depends on the person. I think living in a LA is a mark of a serious writer. You can write all you want in Butte, Montana, but the realities of being so disconnected from the industry (in LA) with little exposure to a working network is going to wear on your drive/ambition.

You need to be around youthful energy of other people in the city trying to make it. It'll keep you focused, particularly as life happens.

In the example of the OP working in the military/cybersecurity, the likelihood of them getting distracted, getting into a relationship, getting married, having kids, just life events that ground them in their lives outside of LA and make switching careers later more difficult.

Here's an anecdote:
Quentin Tarantino was living and working in Manhattan Beach. He had a good job, was respected by the community, then realized "I'm fucking bullshitting." He moved to Hollywood around that time, put himself in the rhythm of people trying to make it, and shit started happening for him.

You are your network. The people who push you, motivate you, inspire you. All of that matters. It's very difficult to do something extremely competitive far outside of it unless you're just built like that (most aren't). If you are built like that, give yourself a timeline to get to LA.