if weight is an issue on a particular day, i take the d810 with a single f/1.8 prime. i find it hard to imagine that any reasonably healthy adult can't carry 1.3kg for a day or a week. when i'm less concerned about weight, i'll bring two bodies, tripods and lots of lenses including f/1.4 primes, f/2.8 zooms, tilt-shift lenses, big telephotos, etc.
It really depends. Quite a lot of my photography involves taking "record shots" of rare birds or unusual birds, like birds in an unusual plumage or hybrids. When I'm out birdwatching, I almost always carry (apart from my binoculars) a scope and a tripod. That's with a lightweight tripod (Gitzo 2541) and videohead and the Nikon EDIIIA about 3.5 kg. Add to that some raingear on longer trips, water, some food, and, in difficult terrain like the Alps, some safety gear, it all adds up to quite a bit of weight, so I have to try to keep the weight down as much as I can.
For birds, even for record shots, I need long lenses. If I were to take a fast DX body (D300, D500) I'd need at least something like the 300/f4 PF with a 1.4x converter. The consumer bodies with their low speeds and small buffers just don't cut it for that kind of photography. Adding to that a 35mm prime for landscapes and so on I'd end up with a minimum of 2kg weight. Quite a few friends use the Canon 7DII with the Canon 100-400mm for that purpose, and that's too much weight as far as I'm concerned, at least for many of my trips. A fullframe body would be even heavier.
So my present setup is the Nikon 1 V1 with FT1 + the 70-300. That's a tad under 1.5kg, including a spare battery and the (excellent) 18.5mm/1.8, and covers my basic needs. Sure, the image quality isn't as good that of a decent DX body, the user interface of the V1 and the EVF are pretty awful, but that setup works OK.
Actually, the image quality isn't that bad as long as I keep the zoom down to ~240mm on the V1 giving me the equivalent of ~650mm. Not ideal, but in many cases sufficient.
Hermann