On Saturday we played in an episode of the Revenge of the Giants campaign. The whole session was mostly a single combat. Sometimes, the party should run away. We didn’t party wipe. Not a single character was reduced to 0 HP. In fact, none of us even used our second wind.
But, if you are 14th level and are in the midst of fighting three level 17 Dire Winter Wolf Iceclaws and two level 18 Winter Wolf Frostbreaths when a level 21 Remorhaz shows up and swallows your Halfling, running really shouldn’t seem like a bad idea. (The Halfling survived.)