I work as in a vaguely language-related field and write books on the side - but ultimately writing is very much a side hustle that earns me maybe £500/month even though I do have readers and am pretty good at the craft. I had a very good period circa 2011-2014 when I would pull in £1,800 a month, but Kindle Unlimited pretty much killed small independents like me, so I'm glad I kept the day job. Even when selling to big publishers (ie Random House), I'd only get a couple grand as an advance and never earn out.
The issue is that there's a few stars (JK Rowling, Dan Brown, etc) - who break out and make multimillions, but those who don't break out eke out some profits (unless their agents and/or publisher shaft them - so many horror stories!) but struggle to make a decent living. The income of full-time writers in the UK is something like £10k. Enough for a mustachian lifestyle, provided you build up your 'stache doing something else. I write a lot less now (demanding job) than I used to, and hope to go back to proper levels of productivity once my 'stache allows it, but for the moment I'm just grinding.