And as far as saying the Lions had time to correct the referees mistake (if there was one)…we can all agree that if the Lions stop the play momentarily to make sure 68 is announced they might as well have also announced to the Cowboys “Hey, we’re throwing it to 68” right?
The NFL rules are designed to be fair to both the offense and defense.
That's why you cannot huddle with one player and then they run off the field and another run on the field before the play gets called because that is illegal substitution.
That is also why you see the referees delay the snap by standing between the quarterback and center for a few seconds when the offense substituted players and the defense has not yet done so.
The reason players have to report as eligible instead of everyone being eligible is because the linemen's numbers dictate to the defense what their role is.
If the non-eligible players report as eligible, then the defense has to be informed so they can change their defense if they want to account for it.
The Lions were likely hoping to confuse the Cowboys by sending three linemen to talk to the referee, but the referee walked over and told the defense (as they should) who was eligible (#70 in this case), so the defense planned for that.
Either through intention or incompetence, they tried to game the system and it backfired.
Had the referee understood #68 was eligible then he would have told the Cowboys defense who in turn would have had someone cover #68 in case he ran a route.
The media and non-Cowboys fans are ranting about the unfair conversion called back, but the reality is that it likely would have not worked had the referee announced #68 was eligible.