If Mr X earns better from a side job when compared to the income from his career option, does the side job still qualify to be called a side job?
Usually career jobs offer opportunities for the individual to reach to upper positions through promotions inside the working field. So even though you earn less from your main job when compared to a side job, you shouldn't give up on it, because on long run it's possible you will have your wage raised and receive extra benefits from the employer, especially if you are an employee on the public sphere.
The important is to work and make as much income as you can. Sometimes we have to earn less now to profit more tomorrow...
While I get your point about acquiring as much as possible, which is something I also support, however, sometimes we shouldn't only seek out money. Don't get me wrong, it's an understandable argument, the world is going through a crisis, cost of living is rapidly rising due to inflation, the pandemic has affected many businesses and sectors, and continues to do so, leading to unemployment, mental issues and financial depression, consequences that we'll suffer for many years to come.
On the other hand, after suffering in a job I do not like (and continue to do so), just because it pays well, personally, if I had the opportunity to do something else, with worse pay, but better conditions, then I'd do it.