From a couple of years ago, there was enough evidence that road racing puts cyclists at substantial risk from OTHER people's bikes.
This is different from MTB which is more of a solo sport, even if done in a group. Also speeds are different.
They could put guards around the discs. Perhaps bash guards around the chainrings too.
Unfortunately, road racing has long been dominated by a minimalistic bare skeletal design since the beginning.
How light can one make the bike and still make it functional?
Yet, one could make an Carbon fiber or PVC/ABS disc guard that would only be a few grams, and could offer substantial protection.
Molded rubber (allowing some flex for mounting wheels, but still giving wrap-around protection).
Mount the guard to the axle so it stays with the wheel?
Will this take a permanent, and non-recoverable injury to a big-name cyclist like Sagan to actually get notice?