The single Lens Reflex was a solution to a problem: How to accurately see what the lens saw. The problem is no more ... Mirrorless have the advantage of simplicity to boot. No more complicated mechanisms to move the mirror up and down... No more bulky prism to correct the view from the lenses..
But in fulfilling one set of requirements, it violates another set of requirements. It's impossible to see what's in focus, because apparent focus is limited by the low resolution of the EVF or screen. So, you have to have additional visual clutter to check focus, such as focus peaking, or zooming the image in. That's fine in some scenarios, but in most active situations, all that stuff just separates me from the subject. Then there's the (albeit short) time lag and smear of the EVF. Mirrorless is designed around the expectation that autofocus works perfectly. But then I have to move the focus points around to make sure what I want focused is in focus--more slowdown. What's supposed to be faster ends up being slower.
Meanwhile, I can focus an adapted Zeiss Sonnar from 50 years ago, that has a really special rendering, using the split-image focus screen on my Pentax 645z, and get perfect focus before I can even find the menu item that turns on focus peaking.
With a sensor the size of a deer tick and the focal length of lens that goes with it, like what's in my iphone, nearly everything is in focus anyway and I don't much have to worry about it. But with a 44x33 sensor, my depth of field might be razor-thin.
(Part of a test series. The flower in front of the boot anvil was the focus point. Pentax 645z, Zeiss Sonnar 180/2.8 at f/2.8, manually focused, of course.)
(Even tighter, despite the sensor that is only 24x36. Canon 5D, Tamron SP 90/2.5 Macro, at f/2.5., again manually focused, of course.)
Timing was critical. In both cases, the breeze was moving the target flower into and out of focus seemingly at random. I had to be able to predict when it would pass through the focus plane and push the shutter button in time to anticipate that.
Rick "give me a good split-image focus screen in an optical finder system any day" Denney