I don't think Switch can maintain the same year over year growth in December because Nintendo closed out 2018 strong. Still expect 2019 to show growth, but not on the level of 29%.
Sony's forecast for PS4 shipments suggests that they won't have many deals, if any at all. Their likely strategy is to tank the rest of the fiscal year to get better draft picks for the PS5.
I agree, December will be hard for Switch to keep up, I still expect over 20m for the year though.
When the herd loses its way, the shepard must kill the bull that leads them astray.