Helmets are always on my head because I think those bones are nicer unbroken and hard., but is there a legitimate concern we'll see an upward spiral of injuries?
Not necessarily. 
Prior experiences with DC and elsewhere have some pretty burly-big data that show bike share customers are more prudent with their personal safety  than owners cycling are, and also that there's a tipping point of auto driver awareness caused by the critical mass of visibile riders and that conditions drivers to watch and anticipate bicycles. Net results? Injury rates DROPPED in DC even though the city is virtually helmet free.