You make some great points, and you are absolutely correct that most artists can’t earn a comfortable living from their art.
But it’s not just that we’re “told” that day jobs are a burden. For many creatives (though not all), working full time IS a burden. Every minute sitting behind the desk, or standing behind the counter, or whatever, is a minute you can’t be writing or painting or practicing. I am filled with this frustration every day. I write and do my creative things when I can, but it’s like I can only squeeze them in the tiny spaces left after work and taking care of a household.
Time is a valuable commodity — we are only given a limited amount of it. True, most artists won’t make a living with their art. So the goal should be to maximize the money earned, and minimize the time working. Artist Inc. calls this creating a “portfolio” career — cultivating multiple income streams that allow you to have more time for your actual art. This can include part-time work, freelance gigs, teaching, etc.
There is nothing inherently wrong with having a “day job.” And I think it’s rather elitist to dismiss artists who have to (or choose to) have one. But there’s a reason most artists don’t want to be tied down to one. It’s just too bad that it’s so difficult for us to have any other choice.