The BNC connectors on the ALEXA 35 are custom 12G BNC connectors that can be exchanged from the outside with a special ARRI tool.
You can see how easy this is to do by watching a video at this link.
Having user replaceable 12G BNC connectors makes a lot of sense. Any connector that protrudes from a camera is always going to be a potential point of failure regardless of how well it is made. If an onboard monitor falls down and it is connected to a BNC port it has the potential to rip that connector out or damage it. An SDI cable could also potentially get snagged on something and cause damage to a connector if the force is strong enough.