There's no way of knowing where those guys would land without the benefit of hindsight. If they really wanted either of those players (would have preferred either, to be honest), they would have taken them with that pick. They rated Harris higher. Trading down would have risked that. And for what? Maybe a 3rd round pick depending on how far they dropped? That is especially true with Foster, as every team between us and Atlanta needed a LB. If he was the target, it would have been terrible to trade down. And this doesn't include teams that may have traded up in response to our trade down and grabbed either target. Only luck would have saved them (à la Tunsil).
And to be fair, people on here want to trade down every year. I honestly think it has more to do with hearing the Phins called more often on draft days so they can get their football fix before the long winter of offseason arrives.