Fares will be down, crowds will be up, delays will be longer. You're more likely to be in the middle seat, or next to someone in the middle seat, or sitting at the gate because you got bumped off the airplane.
All in all, it's a formula for lots of delays and long lines.
It doesn't have anything to do with in-flight safety, but they may cause delays and disruptions. The FAA may go in and inspect and the airlines might not be able to prove that it's in compliance.