Painting within a genre

I always hear that to be successful, you need to paint within a genre. Flowers. Landscapes. Skys. Whatever. Why is that? I do paint a lot of ballerinas and ladies walking the beach. Some people do know me for that. But why is that all I should paint? I have so many things in my head that I want to paint. (And am painting!) Horses, flowers, woodlands. There's a million things I want to paint. If you read this, please leave me a comment as to why artists should paint a certain thing only. I am really curious to know what others think. I realize I can paint whatever I want. It's just frustrating to think that I might be wasting my time painting other things. according to some people.
This is a painting that is currently on my easel. It's not very large, something like 12x15. I am thinking of painting it 40x60. Why is that wrong? I have recently started painting stylized flowers. They have been very fun to paint. So does that pose a problem with my status as a figure painter?  I want to think it doesn't but from everything I read, it's a no-no. I can and often do throw caution to the wind. But what I don't know is what I am missing by painting what inspires me. Your thoughts???