How many games have been won by the team that was, at some point, 10 demigod kills behind?
Yes, comebacks do happen, but my guess is that they are extremely rare.
Lol, I was in 2 games where it looked really bad for us today and still, we turned it around at some point. In the last game I would leave the base only to be whacked instantly by two UBs or an Erebus. (So much for testing a new build against a premade team in live battle...

) Still, we managed to surprise and distract them long enough for the creeps to pop the citadel.
Sometimes you just know it will be a long and hard battle and you plan accordingly with the right upgrades and items. It might turn out okay even if you get slaughtered for 15 minutes.