Aluminum or steel, it matters not. Stripped threads are due to overtightening and/or failure to use a gasket under the drain plug.
Aluminum is weaker than steel, there is no doubt about that. However, it’s perfectly acceptable as long as the threads are not abused.
If they’re going to use the argument about weak oil pans then ask them how many eons spark plugs and oil drain plugs have been screwed into shallow threads on aluminum lawn mower engines.
The pan can be repaired with a thread insert. There is no reason to replace the pan unless it’s catastrophically cracked or damaged. Thread damage is a minor fix; and they should pay for it.