Totally, even if the reason to retreat might not have existed until you just made it up because the plot was about to go to hell. But, agreed, it's a better way to do it and done well, while kinda fudging, gives everything much more depth. (For example, could give you an excuse to bring in an NPC or group that might be able to work with the party in interesting ways. An ally can moderate difficulty AND repeatedly make a long storyline more interesting.)

Still, nothing wrong with "coincidentally" having a strong mob roll a nat 1 and eat dirt to buy a round or two while you figure out a way to put things back on track.