I think Canon has been in the sales lead for a long time. But that is not really the issue. Ford sells more cars than BMW.
I'm a lifelong Nikon user. Just personal preference. Never thought they were any better or worse than Canon.
Over the last few years Nikon has shown some disturbing trends. They seem to have totally misread the DX market. The D200 and D300 were superb, nearly pro level cameras. And then for years, six years?, nothing in that segment with Nikon calling the D7x00 their 'flagship' DX body. I waited and waited for a "D400" to replace the D300. I was ready to walk away from Nikon despite a significant investment in Nikon glass.
I'd be using something else if the D750 had not come along. Mind you, I did not want to got to the FX format but fortunately the D750 was, and still is, a very compelling package. This alone shows that Nikon still has 'it'. But I sold my Dx glass and replaced it with expensive (to me) FX glass only to have Nikon unexpectedly release the D500 which is the pro-level camera I've been waiting for for years. It is a superb camera but it came at the worst possible time for me since my desire was to stay in DX not FX. I'm happy with my D750 but still furious with Nikon's unpredictable and rather idiotic upgrade path.
If the D750 and D500 were not such excellent bodies I'd be even more worried. But I remain worried about Nikon's inability to read and respond to the market. That is a recipe for disaster and the photography landscape without Nikon will be a diminished one regardless of what brand you like. Canon and Nikon drive one another forward.