This is a little cynical, but it's not something they'd want to have known. Suppose the worst case is true and the Webb has a useful lifetime of three or four years. What were they going to do if they'd learned that in 2015? Redesign the entire mission? It was already too expensive. One might say that they should have made the measurement back in the '90s, but the Webb was anticipated to be significantly cheaper back then and the cost of a pathfinder mission would have been proportionately greater.