But first the artist has to get a gallery to show his work.

exactly, I'd imagine artists spend a good bit of their time schlepping around convincing galleries to display their work.

There's a big difference between "massive success" and having a gallery show. Once you have a decent relationship with a gallery, if you don't care so much about money you can just let the gallery handle that side of things.

A friend of mine has been a professional painter for the last 50-ish years. He makes a couple trips a year to the gallery to deliver paintings. The gallerist tries to take that chance to suggest a few motifs that seem to sell better, but doesn't get too pushy.