40 times aren't everything, granted. But the 40 does measure whether or not someone has a single elite trait (long speed) which unquestionably matters at the WR position. (Again, don't hear me saying "IT'S THE ONLY THING THAT MATTERS!!!")
For every WR prospect with an elite 40 time who flames out, there are at least 10 more with pedestrian speed that don't stick in the league because they don't have any elite traits.