In that case, I'm saying there would only be up to 66 (not 70) paying customers in the jetway. This still leaves 4 available seats for the deadheading United employees.
In other words, don't render those 4 extra United employees "invisible" to the reservation system only to later make them "visible" when they finally show up at the gate after all 70 passengers are already in their seats. United knew they needed those 4 employees in Kentucky hours before customers boarded #3411 in Chicago. The issue is that they had zero empty seats on that flight so they had to bribe/force 4 "volunteers" to give up their seats.
If United's computer system doesn't have enough sophistication to set aside plane seats for employees that have higher priority than paying customers, they deserved to have the situation escalate into a Mexican standoff between the gate agent, the seated customer, and the Chicago police.
If you're talking about a rare emergency circumstance where United's operations center doesn't know they need those 4 seats for employees until after all customers are boarded, yes that can still happen. However, that wasn't the case for flight #3411. The 4 employees were "running late" to appear at #3411's gate. This means they were not a "hot standby" crew at the airport's lounge. Instead, it meant that these employees were are either on an incoming flight or were stuck in traffic driving to the airport from home/hotel. Either way, United had plenty of time to reserve 4 seats on #3411 for their higher-priority employees but for whatever reason, they didn't do it.
All 70 passengers are in the jetway or on the plane. Say, 40 are on the plane, but 30 are still in the jetway. At that moment, United discovers that it needs 4 crewmembers on that plane. They have 4 crewmembers sitting around in a crew lounge in that terminal of the airport. They can walk to the gate in 3 minutes. 3 minutes from now, some of the passengers will still be in the jetway, waiting to board the plane. Now what?
It's not enough to just improve the computer system. The timing window still exists, even if it's smaller. What you need to fix this is some kind of business rule, like "You can't deny boarding to a passenger who's already in the jetway, no matter how badly you need their place for a crewmember".
What you're describing is a race condition, to use the term from real-time software. By updating faster, you reduce the timing window, but the problem can still happen - the passengers are all in the jetway, but the 4 crewmembers are in the employee lounge and just a 3 minute walk from the gate. They can still make the flight before all the passengers are out of the jetway...