I'm not against making it extendable per-se. However:

1. That almost always means much more than simply making a constructor public. A Class/contract needs to be designed with extensibility in mind, and this Class was not. E.g., if this is to be extended it makes sense to define #buildPropertyAccess as protected. Etc
2. I'm a bit leery of the timing simply because all of this is going through such a dramatic change in 6 - I think we should be looking to minimize the "migration guide", not seek to grow it.