Just so
Most of the players in my campaign, all of my own PCs, were never too proud to take to their heels when the opposition was clearly overwhelming. In fact, a good bit of thrilling adventure went into some of the ensuing chases. When Mordenkainen and bigby met their fate at the hands of Rob's super iron golem, it was bacause they hadn't the means to escape quickly, not that they didn't wish to beat feet
As a DM I made lemonade out of the "lemons" of the new players' reluctance to flee, to stand and die before a foe that was trashing them. That is how the Old Guard Kobolds came into being, and they have grown in both individual ability, numbers, and organizarion so they now pose a meaningful threat to PC parties of 4th or 5th level. Sadly, a group of such PC that were veterans took ost of them out, but iw was a casual play session, so I won't count it against the little humanoids
Cheers,
Gary