I agree that "always" is a slippery slope. I just wish it were that easy.
We've all had this happen at one time or another...So for example you have 11 break the huddle (or maybe no-huddle) and head to the line of scrimmage. Thinking the offense only has 10 on the field, a coach sends the 12th player out there. The coach or 12th player then correctly counts and realizes he's #12. So he immediately turns and runs back toward the sideline. He doesn't get off in time at the snap.
Does the following sound correct?
Scenario 1: If all 11 had set for one second before #12 entered the field, then he didn't get off in time then we have a live-ball illegal substitution (or illegal shift?).
Scenario 2: The offense is hustling to the line (not set) when 12 comes running out, then if he doesn't get off in time we have a dead-ball false start, even if the original eleven got set at the line for once second before 12 made it off.