I note that it isn't particularly difficult to construct scenarios in which a given party would have a nonzero probability of never being returned to their own time. I shudder to think what might happen to them in that case.
Really?
Even if the party is completely incompetent it still seems there must be at least some chance that by a series of extraordinary coincidences they will win. Regardless of how low your chance of victory is if you have enough tries you will win eventually with probability 1.
The current Goblins Comic is relevant.