The NFL is in a constant flux of change. Rules are changed, added, altered, and examined consistently. This does not mean changes are made but are looked at when brought up.
It used to be that players never left the field and only three substitutes a game was allowed. This was an old soccer rule, and so American Football simply utilized things that were familiar and easy to implement. In 1950, free substitution became a new rule.
A punt at one time could be recovered by the offense and gain a new set of downs.