I think that Canon and Nikon have been resting on their laurels, in large part because they have the photojournalism market sewn up. They're the undisputed champions of a small and highly visible market, but they have been overtaken in almost every other respect. They don't make the best sensors, they don't make the best lenses, they don't make a compelling mirrorless camera.
Canon and Nikon don't have the aspirational appeal that they used to. Their prestige has been badly undermined by the Sony/Zeiss partnership - the best sensors paired with the finest glass. Sigma used to be the cheap option, but they're now producing some of the best primes in the world. It used to be that serious photographers were either Canon or Nikon people, but now they have a lot more choice.
Nikon just doesn't have the money to compete, I'm friends with one of the Nikon glass people, as in the people who make the glass, he's just depressed each time I come over with some great Canon lens (I showed up with the 400mm f4 DO which is identical in size to the 200mm f2, he was stunned they could make a 400mm that small with optics that good).
I whole heartedly disagree that Canon is coasting. Years ago they knew that they would make a 50Megapixel camera and they started redoing their lenses to resolve details at that pixel size. Go look at the mark II lenses, the 70-200, the 300/400/600. Go look at the bodies that do 50MP, the landscape people love them.
Canon is investing heavily into bodies and glass.
That said, I've got the Sigma 85mm ART and have looked at other Sigma glass. Couldn't be happier with the optics, they are still behind in the auto focus speed. If that's not an issue then Sigma is killing it and that's a good thing.
Canon and Nikon don't have the aspirational appeal that they used to. Their prestige has been badly undermined by the Sony/Zeiss partnership - the best sensors paired with the finest glass. Sigma used to be the cheap option, but they're now producing some of the best primes in the world. It used to be that serious photographers were either Canon or Nikon people, but now they have a lot more choice.