Are you suggesting, Eddie, that unless the Lisbon Treaty contained an escape clause, no nation would ever be able to leave?
All A50 does is provides the EU with a two year negotiating window to screw all it can out of the escapee. Were it not there a member could simply say "that's it, we're off" and that would be that. Members signed the Treaty knowing that the way to leave (bar breaking the terms of the treat, which no member wold do) was via A50.