@Dries, on #8, I agree. I started off with the assumption that in D8, if a table is not a part of the config or some other API, it probably doesn't have schema defined as well. I intend to write another source specifically for tables supported by schema and you wouldn't need to define id_fields or fields for that.
As for the middle ground, I am fine with it. I started with core because I had forgotten about migrate_plus and I did not want to create a module just for these two destinations. I know core doesn't use it and chances are that everyone writing migrations will be using migrate_plus anyway.
As for the API argument, I agree theoretically but in practice, I have seen situations boiling down to copying tables too often. Like I said, almost every non-trivial migration I have worked on has used this table destination, which is part of migrate module for Drupal 7. That said, I agree it is more of a developer's item.
Thank you again for all your feedback. I will keep this open for another day or two and if there are no more comments here or comments suggesting moving to migrate_plus, I will work on patches for that.