There's a difference between something working by design vs. working by happenstance. I would argue using a gTLD that isn't supposed to be registered is working

There's a difference between something working by design vs. working by happenstance. I would argue using a gTLD that isn't supposed to be registered is working by happenstance whereas the proposed .internal would be working by design. As an example, some (terrible) ISPs intercept NXDOMAIN records which could leak into misconfigured networks that are squatting on unregistered gTLDs. This behavior would be far worse if they did this for the proposed .internal.