Well whitebeard's a dumbass first of all, there are plenty of routes he could have gone down that didn't involve kidnap.
But I do still think that kidnap wouldn't have really worked, to begin with his movements are under constant watch by the WG they would have seen a plot a mile away. And if he sent an underling, the CD has a buster call they can use at any time to counter it. Also, Ace's death day was already set, he would have been on a time limit which he needed to prepare for the war and mobilise all his allies so he didn't really have time for a plan that may or may not work.