You can see it too -- and even on your monitor. I shot it for you. I just happen to have a best example of a retrofocus lens designed for a DSLR. Designed for FX with a 90 degree angle of view I have a Zeiss 21mm f/2.8 Distagon. Arguably one of the best in class in that type of lens. The folks at Zeiss are no slouches and they make their best effort bar no expense. What that means is if you have a similar brand __________ lens odds are it's not as good.

So I set up a tripod in front of my mailbox -- window to the side and brick wall behind. I put an FX camera on the tripod with the Zeiss Distagon and did my best to square up the camera. I didn't get it perfect but I did well enough to show the difference.

Then without moving the tripod so much as a mm I switched cameras and put up my Fuji XF 14mm f/2.8 lens. Same angle of view but designed for a DX sensor. The two images are then very close in content. And what you'll see if you compare them is what we call mustache distortion in the Zeiss lens -- that's pincushion distortion on the edges with some barrel in the middle -- very hard to deal with in software. The Fuji lens on the other hand is free of distortion. All you see is my failure to square up the camera (it's not perfectly horizontal to the wall).

The Zeiss lens although some years older cost a whole lot more than the Fuji lens. Why the problem with the Zeiss lens? The DSLR mirror plain and simple. If they didn't have to design the lens clear of that floppy mirror they could remove the distortion like Fuji did. The Fuji lens of course is on a mirrorless camera and is not a retrofocus lens like the Distagon.

It may not be a difference that matters to you or a lot of other folks. It's great that we're not all the same. It is a difference that matters a whole lot to me.

Just a little food for thought

Looking completely dispassionately which is better photographically – for image quality?

First the significant differences (Comparing APS-C and full frame only):

DSLR bodies are bigger and heavier than mirrorless bodies.
DSLRs take many more photos per battery charge than mirrorless
DSLRs have mirrors :)
Mirrorless don’t have mirrors :) :)
Mmmm – can’t think of any other significant differences.

Second important similarities:

Both have sensors
Both have viewfinders
Both have LCDs
Both have lenses
Mmmm – can’t think of any other important similarities.

So how does any of this make a mirrorless better than a DSLR?? Well IMHO it doesn’t, it can’t and it won’t as far as image quality of photographs is concerned. Excluding the photographers skill levels and the many personal preferences involved, the image quality of a photograph is directly proportional to the quality of the lens and the sensor.
Mirrorless do not have better sensors than DSLRs. Mirrorless do not have better lenses than DSLRs. So photographs from mirrrorless cannot have better image quality than those from DSLRs.

Most of what you say is true but not all....

Most ML sensors are of a newer design - for right now anyway - making them BETTER

Most ML lenses are of a newer design and with a bigger mount resulting in BETTER lenses/imaging !

All ML bodies have an EVF which facilitates correct exposure, and other pertinent data in the VF - also facilitating correct settings

ML cameras have no mirror / slap/induced, vibration, shutter lag, noise or VF dimming in low light.

ML cameras are capable of eye AF !
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I have just reviewed eight pages of comments on this thread, and that has left me totally exhausted. I'm a deeply technical guy who during most of my life has pursued the holy grail of perfection in most things I do. That would normally find me moving to mirrorless for some of the reasons enumerated in this thread.

The only reason I am commenting on this thread at all is because I am always amazed when someone takes a basic photo of a brick wall with a few un-intricate objects and thinks that constitutes comparative proof of superiority of one type or another. If I were doing a comparo of this type I would use something like one of the celebrated "lens test charts" that might better show some comparative superiority one vs the other.

Not saying mirrorless are better but they are different. You missed a couple of significant differences:

1 Mirrorless don't have optical viewfinders
2 Mirrorless don't have mirror slap vibrations
3 Mirrorless don't need focus calibration because the focus is determined directly on the sensor all the time
4 Mirrorless cameras are less tiring when hand holding with long lenses (800 mm) to shoot wild life.

Also I'd note that all lenses are not currently equal. The new line of Canon R mount lenses which can only mate to their new mirrorless R series cameras have been found to be sharper than their best previous Pro line of lenses. This may indicate the lens design process allowable with mirrorless cameras makes possible new and improved sharper lenses. Perhaps not better image quality but potentially sharper images.

I may be guessing but it sounds like you've not used a current mirrorless camera. Use the tool that you want to perform the task. I particularly like the "Eye Focus" feature and in camera focus stacking.