If the bar for "can support himself" is suitably low, someone could be an artist, immigrate to the US to work as an engineer or accountant or nurse, and be an artist. Once he can support himself through art (and is likely a citizen), then be a great artist full time.

You have a really sad view of what "art" is, if you think "oh, he can just be a <insert equally time consuming profession>"

For a period of time, I absolutely do think someone can do some other career in addition to art, rather than receiving public assistance.

What is the point to making self-sufficiency an externally-enforced requirement?

If are are going to be deprived of the means of subsistence by state force, we shouldn't expect them to be able to support themselves.