Originally posted by pakinjapan I would say “weight”. One factor pushes people to mirrorless are the weight. [I agree, size is not so important any more. my K3 is no way a big camera, IMO]
Rather than a hybrid body, a dedicate EVF unit could be an answer here, but some (at less me) will say, I can just use the rear LCD, why pay for a dedicate EVF. lol
A mirrorless APS-C camera with K-Mount and a viewfinder will not be significantly smaller than a K3. Much of the size of the camera is dictated by the registration distance the large battery and in body image stabilization.
The compromises Fuji made to get the X-T cameras as small as they are (and they are quite svelte) would be unacceptable to many photographers. The battery life alone would be enough to chase a lot of people away. If you tend to spend a lot of time with the camera to your eye, you can expect as little as 50 pictures to a battery. The most I can squeeze out of my X-T1 is about 250, and that is by being very aware of how I am doing things to ensure i am conserving battery power as much as possible.
One can say, yes, but one can carry spare batteries, and this is quite true. I have to carry 4 -6 batteries, to give me the same shooting life as one Pentax battery. This does cut into the weight advantage, and now one has to keep track of more strew in a camera bag. Batteries also seem to die at the least convenient moments.
There are advantages to a heavier camera as well. It is much easier to handhold a camera steadily if it weighs more.
It is also easier to handhold a camera steadily if it is close to the body (like pressed against your face looking through a viewfinder) than if it is held at arm's length looking at a rear screen.
With my X-T1, for example, using the viewscreen rather than the viewfinder costs me about 2 stops of low speed shutter duration because the camera is now less stable. I discovered this because the EVF does tend to give me a headache (one of the things that EVFs need to catch up on compared to OVFs) and I was trying to figure out if arm's length hand holding was a viable approach (it isn't, at least not for me).