how to respond when asked "why don't you get a DSLR?"

botw wrote:

I think you are a little optimistic about what sort of camera knowledge people have.

As for many others, dslrs - at least the nice ones - still have many advantages over mirrorless camera. Mirrorless cameras' main advantage over them is size, which is not an insubstantial advantage. In the end, this is a complex question and one I wouldn't get into unless you had a real desire and willingness to discuss with the person.