Total agree. However, I would like to add "Form (size, weight)", "Function" and "Econonics (price)" also play a very important part. In the film days, 35mm full frame SLR is not the most propular camera. Small view finder 35 mm camera was the most popular camera. That is due to lower price and easier to carry ( small size and light weight) even with limited functioality. There is no doubt that the Leica M series is the best 35mm film camera. But for functionality, It cannot complete with SLR unless expensive and clumsy attachments are used. Therefore it became a limited production camera for certain user. Rolleiflex is an excellent camera. With waist level finder and shutter release at the front of the camera to give the photographer the perfect angle for full length body shot. Due to higher price and fixed lens it just cannot survive. Hasselblad is a 6X6 SLR. Again, high price and heavy weight limits it to become a specialty camera. The examples can keep on going. So I will stop hereI keep saying similar things (IE use an slr body type, with an EF mount - so no one has to use silly lens adaptors or wait while each and every lens ever made gets resigned to fit the current mirrorless mold). But, this is where mirrorless has its downfall, it seems like the biggest proponents for mirrorless want their cake and want to eat it too. All the bells and whistles of an slr, in a package smaller than the A7, but smaller with smaller lenses and of course, a magical battery compartment that can fit 2 1dx batteries...+1
when it comes down to it...it really is about form factor. there was another post somewhere here showing the first digital camera's, goofy looking things, the first idea was that cause it's new it should look radically different...high tech...result, they looked like a joke and weren't taken seriously until digital camera's started to look like regular cameras.
which is why i feel that mirrorless may just be a cool for now, trendy product.
You can't ignore ergonomics.... Why was 35mm so popular back in the days of film? We had a lot of standard film sizes to choose between. Myself, I seem to have used from tiny scraps of film in instamatics to a friend's 8x10....
35mm was the sweet spot.... it was the combination of ergonomics that made it a good size to hold, yet allowed a range of lens sizes with reasonable image quality... yes, you could go bigger (I did hump around an 8x10 ) but by going bigger you needed insanely large lenses to get a decent field of view with animals, birds, and other distant objects.... There is a very good reason why Ansel Adams shot landscapes and not BIF 35 mm was a good general purpose balance point.
So now we have gone digital. People are still the same, so the ergonomics remain the same... the laws of optics are the same, so other than better materials and more precise manufacturing, lens are essentially the same.... The sweet spot for size remains the same...
Put a 50 year old SLR and a brand new DSLR on the table. It is obvious that they are both cameras, and with the exception of a preview screen on the back and the relocation of a few controls, they are not really that much different..... ergonomics strikes again!