Or two A7r bodies, a heavy tripod and C1 cube head, 15kg of lenses and a 5Ds, 200-400 and monopod for those two days of wildlife shooting in the middle of a three week landscape trip, as the case may be...
Mirrorless doesn't necessarily mean light weight.
Agreed: clearly, when one needs big, fast lenses, body bulk is rather irrelevant. On the other hand, one thing that is pushing many photographers towards using EVF cameras at least part of the time is the great increases in sensitivity (reduction in noise at a given ISO speed; higher usable ISO speeds, getting the same usable ISO speed in a smaller format) compared to sensors from some years ago, and even more so compared to film. Because one way or another, this leads many of us to sometimes take photographs in ways that deliver far less light to the sensor, and hence less light to the OVF. An EVF camera can keep the VF image as big and bright as ever by increasing the amplification (allowed by the lower noise in the newer sensors) but with an OVF, the image must get dimmer, or smaller, or some mix of both.
The situations I am thinking of include:
1) Shooting in lower light with a given lens.
2) Shooting in the same light at the same shutter speed with a higher ISO speed and higher f-stop, allowing the use of a lighter lens through the lens having a higher minimum f-stop.
3) Pushing telephoto reach further by using the same-sized lenses with a smaller format (or using them with heavier cropping, which I count as using a smaller effective format).
4) Pushing telephoto reach further by using a tele-convertor or a longer but slower lens, offsetting the f-stp with the higher usable ISO speed.
I am often in case (3): I ver owned a lens longer than 300mm for my film cameras, but the relatively lightweight MFT 75-300/4.8-6.7 gives telephoto reach as good as (actually better than) a mythical 150-600mm on those film cameras. It would not work well on a Four Thirds SLR, because at f/6.7, the VF image would be uncomfortably dim or small, depending on the OVF magnification.
Of course, one can also use rear-screen Live View on a DSLR in this situation, but the disadvantages of doing that, especially with hand-held long lenses, have been much discussed around here!