I don't think there's anything inherently wrong with PHP. It has it's shortcomings, but if you program carefully I don't believe those become an issue, especially if you've found that PHP is sufficient for your current issues. I think PHP's biggest strength is that PHP has an abundance of free libraries, and works on virtually every OS, etc...
However Python shares those strengths, and in many ways is considered a superior language. Switching may even spark your creativity about how to improve the site.
I think what you need to consider if you can afford to spend the time redoing your site in Python, and learning Python sufficiently well to really get the benefits out of it. If you can, you may also want to look at other emerging web-dev languages and see if other languages suit you better. But if you can afford to do it, a switch could be good for you.