Four years ago the Sony A7 system was a joke when I tested it, and even though I had the money, I was unwilling to put up with the problems that the first generation of A7 cameras had. Today, Sony has greatly expanded their lens offerings and has corrected almost all of the faults in their current a A7 offerings. Nowadays Sony represents a formidable opponent for Nikon and for Canon.
We will soon see what kind of sales traction the Z series aps-c camera offers. Both Canon and Nikon have seen small-sensor mirrorless cameras that yielded pathetic sales. I think that aps-c might be a good Market for mirrorless .The Nikon 1 series and the Canon M series have both been disappointments for their parent companies, at least in North America. Canon has not seen fit to import a number of M-series lenses that are sold in other markets, And the Nikon 1-series was such a sales disappointment that Nikon decided to scrap it. I think that aps-c might be a good place for Nikon to replace the 3000- and 5000- series DSLR cameras with mirrorless offerings, possibly even using the F- mount.
The thing is Nikon is both a camera and a lens maker, and I don't think sticking with the old F-mount offers Nikon itself much increase in profits. The Z-mount means people will buy new lenses.
Both Canon and Nikon decided to enter the full frame mirrorless Market at the same time, about four years after Sony had entered it, and after Sony had carved out a solid market segment. It will now be up to Canon and Nikon to see if they can make inroads against Sony. I actually have a bit of faith in Canon and Nikon in the camera Market... they are after all longtime members of the Camera & Photo Market, while for seven years in a row Sony failed to turn a profit.