A framework shouldn't (and most all the popular PHP frameworks out there, including Symonfy adhere to this) matter when your business logic is decoupled from from your framework. Yes, you might have to rewrite some constructors. And some frameworks have different features, but can still be carried over if you need it. You aren't beholden to a single code base.
For example, I can easily move my models from Zend to Symfony without rewriting a thing. Decoupling is a wonderful thing.
So, you have this code base you are using, these libraries that are already written, and tested, and working. Moving to Ruby/Python would require that they all be rewritten, and appropriate libraries be replaces.
Moving to another language isn't a weekend endeavor when you have a workflow and toolset built up over a 10+ years of development in another language that, in that domain, accomplishes the same thing.
For example, I can easily move my models from Zend to Symfony without rewriting a thing. Decoupling is a wonderful thing.
So, you have this code base you are using, these libraries that are already written, and tested, and working. Moving to Ruby/Python would require that they all be rewritten, and appropriate libraries be replaces.
Moving to another language isn't a weekend endeavor when you have a workflow and toolset built up over a 10+ years of development in another language that, in that domain, accomplishes the same thing.