I think we're talking about different things here. My intent is not to rain on your parade. I you want to believe that Susan or Romana or other Time Lords are still out there, that's of course fine with me. I don't really care for the in-universe situation, to be honest, because the Doctor Who universe doesn't actually exist. The only things that exist are writers, what they have written and what they intend to write in the future. For now, the Doctor is the Last of the Time Lords. It means that there are no other Time Lords, otherwise what is supposed to be tragic would just be a sitcom-level silly misunderstanding. Clearly, that is not what was intended by Russell T. Davies & Co. And yes, of course it may not be a permanent situation. Nothing is permanent in serialized fiction. Any writer could bring the Time Lords back in any kind of ways, and whether they're dead or not doesn't change a thing. In an anything-goes fictional universe like the Doctor Who world, nothing prevents anything from happening. But for now, that has not happened, and from the show's point of view, the Doctor is unambiguously the last of the Time Lords, i.e. the only one.