Oat milk isn't rare outside the US. It's a bit harder to get in French coffee shops, but not that much.

This is just France being French. Especially because Toulouse is a small regional airport.

Personally, I agree with you. Oat milk just meshes remarkably well with the general flavor profile in coffee. It's thick, has the right general umame taste, and the texture is a lot less offensive than almond milk.

If I'm making a sweeter coffee, then I'll go for coconut. But oat is definitely superior in regards to complementary tastes. Dairy is versatile (and, IMHO, the only right way to make a properly foamy capuccino), but there's just something in oat milk that just makes coffee feel like a sturdy, rustic, wholesome drink instead of a delicacy.