The drain plug should not fall out in a day, week, month, decade or millenium. Sheer common sense says it’s in there for the duration until the next service unless someone ham-fisted jackass botched the oil change.
Are millions of customers supposed to be responsible for constant checking of the drain plugs? Not iMO.
If one calls an electrician out who botches an electrical repair which in turn burns the house down is it the responsibility of the homeowner to double-check their work? Again, no IMO,
Personally, I don’t think there’s any way the dealer wins this in small claims court unless the judge and dealer share the same tee times…