I am not an electrical engineer and can only provide anecdotal information.
The information I can offer in answer to your question is this: There are no AC electromagnets that are used in access control applications as electromagnetic locks. There are a very few AC electromagnetic coils used in electric strikes. An AC electromagnetic coil changes polarity at a certain frequency when energized. For example, an AC electric strike used with a step down AC transformer reducing voltage from standard 115v line current in the United States changes polarity 60 times per second, causing the strike to buzz a steady b flat. Because an AC electromagnetic coil vibrates at 60 cycles per second, it cannot be used in a fail safe or continuous duty application because it will buzz constantly until it burns itself up with the friction of the solenoid plunger.
From experience, I have known DC electromagnetic locks to be in operation for 20 or 30 years, so your assumption that they do not weaken over time may be correct. More likely they weaken, but not significantly enough to affect their performance or set off their own bond sensor alerts.