Let me start off by offending half of the forum and say I hate the game Risk. I mean you are either winning the whole game or your losing the whole game.
I really have to disagree with you on this. I once won a game of risk after only having two teritories left in the European continent. After some turns building up my army again, I was able to win the game because I had to wipe out a player. That player was getting beaten up by one of the others had was at the end in almost the same situation as me, so with a few lucky rolls and within one or two turns I could finish of that player and won the game.
Now, to answer your question, as some of the others already mentioned, I think the various victory conditions will make sure that there is always a chance to come back from a seemingly hopeless situation. Now, from the beta it looks like it will be possible to pick and choose which victory conditions will be available, so if you only enable the conquest one, you might still feel unbeatable after conquering half the map.