If they had a replay in place for that game, and the Saints ask for DPI to be reviewed, maybe they get it.
Then the Rams could say "wait, Donald's facemask was shoved and held! Now we want that reviewed!"
The reviews would become interminable, OR there would always be one more infraction that fans say wasn't detected or adjudicated properly.
The problem isn't the system, it's the tendency of humans to want fairness (for themselves), and to have trouble dealing with imperfection.
Ramily!
