Lifetime of, and repairing, an aluminium AC condenser?
Welding of aluminium to copper is frequently done by pressure welding. During that process both materials are pressed together under very high pressure thereby creating a durable bond. Such welding can not be done by a service engineer. So for him the only option is to select two materials of the same kind. Copper tubing and also copper condenser and braze the two together.

Therefore the technician is right.

As far as the expected life of the aluminium condenser is concerned. If the environment is more or less neutral, a long life can be expected. In your case you were just unlucky the joint failed.