I am not sure the NFL was wrong. The pro cap reduces concussions but does at least theoretically increase the risk of neck injuries.
There are no easy answers.
Considering the evolution in teaching how to tackle as well as Targeting penalties at lower levels of football, how would the Pro-Cap on it's own increase the number of neck injuries anymore than the typical helmet? Are there test results? IIRC, it was essentially a piece of virtually no-weight Styro-foam on the outside of the helmet. Not everyone can afford a SpeedFlex and there is little data for those as well.
If and when my son wants to play football, I am far less concerned about any single concussion he may suffer, than I am of the multitude of sub-concussive events that would go unreported. a ProCap would conceivably put a significant dent in those.
If there were an easy answer,It'd already be done. I don't think any one single policy change will solve the concussion problem in sports, but every little bit counts and as long as that little bit does not materially change the game, it should be considered. Just like other worldly issues, the shell would probably not not solve the issue 100%, but that does not mean it should be completely dismissed out of hand. That is what the NFL did in 1996.