I didn't start carrying less weight when I switched to mirrorless; I started carrying more cameras and lenses. Now I rarely waste time changing lenses, I almost never have to say "damn, I left that lens at home!", I can have a choice in camera capability, size and output, and get to use cameras that are smaller, more maneuverable, able to get in places and positions a larger camera can't, and have the most advanced user interfaces of any cameras going around.
So, the choice to use these types of camera doesn't just have to be about not being motivated enough to carrry a decent sized bag of gear around, but whatever way you look at it a mirrorless camera is more likely to get the shot, a shot, any shot than a big DSLR.