It’s a real “I like pancakes” moment here.
No one said the waiter should run to the store. But making a reasonable request for a reasonable ingredient for a coffee is… reasonable.
No one is sending the waiter anywhere.
That’s reasonable according to your standard. That’s not reasonable in most café in France. You don’t need to know that but you’re suppose to understand what’s not reasonable to expect from not seeing it in the menu.
If I can get oat milk in my coffee in fucking eastern Europe I’d think expecting it in France isn’t too crazy.
But then of course French people gotta fail the “Get out of own anus challenge about anything food related” on the beginner level.
You could get free-lactode milk and probably some plant-based milk in some place but no. Oat milk is not a staple in café in France.