This is a perennial discussion when it comes to the Grand Tours, and nothing changes. Barriers aren't always the answer. Some idiot scaled the barriers at the finish on the ITT a day or so ago and ran out to the line. Remember when some lady reached out over the barriers and grabbed Cav's bars when he was in a full tilt sprint at the finish? You can't stop stupid.
What they could do, though, is use more motors on the climbs to clear the crowd back ahead of the leaders. However, that has consequences, like increasing the chance of a spectator/motor collision, having the motors pace the riders because they are so close, and taking away views of the mayhem that many spectators consider sacred and should never be dampened. The bottom line is, rider safety has always been secondary for World Tour promoters. It's all about selling the circus.