How do you harvest perennial onions (Egyptian) so not to destroy the plant for years to come?
Never harvest all of them.
Leave some to go to seed and when the seed heads fall over they will grow new plants.
Also the plants form small bulbs around the main bulb. When you harvest if you leave some of these small bulbs in the ground it will also form new ones. When I pull mine out invariable some of those small outside bulbs stay in the ground, cuz they are torn off in the act of pulling. If you dig them you will want to throw the bulblets back into the hole.
They also spread from the bulbs splitting. Unless you harvest everything the onions will take over whatever you allow them too. You should have no problems having them for years to come.