General opinion is that a side job is one that supports the main income of an individual, often times perceived as something that brings little money to add to what you make from your day job. 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?
Good topic and question you've got here, in my own opinion, it's very possible that side job can become main job (which is also known as day job) if the side job happens to bring in more money than the day job does, for example, some one already mentioned youtubers, I have friends who take streaming on YouTube as their day job and are earning a good living out of it, this set of people had what they were doing before either they found YouTube or someone introduced them to it.
Also myself as an example, I was into public transportation as my day job, along the line, I discovered and got into crypto, later on, crypto started paying me even more than I was earning in my transportation business, I had to later on take crypto trading and investing as my day job and make my transportation business a side job, and I plan to quit the transportation business soon though as I have plans of taking my crypto journey further to the next level.