Well stated. Cisco's docs go into great detail on this - they've spent a lot of time thinking about how to address the issues in plain ol' BGP with SoBGP (Secure Origin BGP), and they concluded that determining a BGP speaker is authorized to announce a particular route is impossible in a functioning internetwork.
http://www.cisco.com/web/about/ac123/ac147/archived_issues/i...
and
https://web.eecs.umich.edu/~zmao/eecs589/papers/draft-white-...
provide more detail on this.