There is a lot to consider when cutting over to a new switch. This video only shows disconnecting the subscriber side of the old switch. They also needed to consider what needed to be done to the rest of the telephone network, what needed to be done to supply power to the subscriber lines, and what needed to be done to make the old IDF safe to take apart.

At that time, telephone call routing information was exchange-level only (the c and x part of ccc-xxx-xnnn). When you dialed a number, your exchange only looked at the first part of the number to know what exchange to send the call to. When they cut over from one exchange to another, they had to give the upstream switches new configuration equipment for all the affected phones at one time. This essentially isolates the old switch from the entire rest of the phone network - it becomes unable to handle anything but local calls - it would not even be able to handle 911 calls. To prevent the old switch from trying to respond to someone lifting the phone off the hook, they cut the cables. I don't know if the subscriber line power was supplied at the MDF or IDF, but either way, this would also have been a factor - each pair of those 35,000 lines might have had 48v power on them.