There is also the possibility of converting our application over to the latest, hottest framework. Although switching to a public framework is a little bit like an application rewrite, it should be much easier now that we have a well-separated series of application layers. We should be able to determine which parts would be ported over to a public framework, and which are merely incidental to the operation of our particular architecture.
I advise neither for nor against this approach. I will only point out that, in the course of modernizing our legacy application, we have essentially built our own custom-tailored framework. We have done so with what is probably a more disciplined and rigorous approach than most public frameworks in PHP land. While we gain the community that goes along with a public framework, we also gain the baggage of the framework developers themselves. These and other tradeoffs are beyond my ability to judge on your behalf; you will have to decide for yourself if the benefits outweigh the costs.