This happened quickly. After Sony was able to claim the market for mirrorless full-frame cameras for itself in the last few years, the market shares seem to shrink from nearly 100% to 67% within a few weeks - at least this is true for Japan.
BCN Retail has released figures for the Japanese market for the last few months. And here you can see a sharp drop since August. Responsible for this are the newly introduced Canon and Nikon mirrorless full format cameras. Especially the Canon EOS R as well as the Nikon Z7 and Z6.
According to this, Canon already comes to a remarkable 22.1% and Nikon to 10.4%. How these numbers are to be interpreted is not yet completely clear. One should keep in mind that this is a growth market.