I think Canon at least will hold strong on their market share. At least for the low end SLR market and the big DSLR pros.
It's hard to beat the old image of SLR = better and you can get an entry SLR really cheap with a 50/1.8 vs what you can get in a mirrorless. As for Pros, those 1D users will hold on to it because of their glass and Canon's roadmaps for glass is relentless. I'd say within 5 years they would have everything a pro needs. Sony E-mount has been out for longer than that and ultimately there are still holes.
Nikon, not sure where they are going, whereas Sony seems to push Body tech first, now glass. Canon pushing Glass first, body second. Nikon seems to be pushing evenly for both at the same time. Mediocre body and mediocre lenses.
That said, I have a friend who is a wedding tog that shoots Nikon, he bought both a Z6 and a Sony A7III in the same week to test them out and he has returned them both to get a D850 to replace his D750.