Let me think...
The first photograph I ever made was with a mirrorless camera. The camera belonged to my dad. I still have it. A Konica I 35mm rangefinder camera with a collapsible Hexar 1:3.5 f = 50mm lens.
My very first camera was mirrorless. I still have it. A Kodak Brownie Hawkeye Flash 620 film camera.
My current camera inventory includes both mirrorless and cameras with mirrors.
They all make photographs.
What's the big deal?