This is going waaayyyy

but...
You're living in the photographic past where there was growth. Not that DSLR aren't going down, just that neither of them are moving up, and haven't been for a while. Technical details below:
In 2018 there were 12,233 additional non-Reflex* shipments than in 2017, a growth of 0.03%, not even keeping up with population growth, and with new models and channel stuffing, sales (not shipment, sales) volume fell about 14% for Mirrorless, and over 25% for DSLR, with hundreds of thousands of mirrorless bodies sitting on shelves compared to a year ago. Once we remove Medium Format, the peak year for 135 and smaller mirrorless camera sales so far was 2016, and the peak year for shipments of 135 and smaller mirrorless was 2017. Peak year for DSLR sales was 2011.
In Q1 2019, year on year camera sales have fallen again by just as much as they dropped in the entirety of 2018, including both categories. That's a four times increase in market contraction rate, which is why the camera makers are scrambling to sell anything they can, with their highly profitable and capable $2000-$3000 bodies are seeing the biggest discounts.
But since you're asking about trends, if the 2011-2018 trendlines, or the 2017-Q1 2019 trendlines continue, Reflex* (including DSLR) will be smaller than Non-Reflex* (including Mirrorless) in 2020 or 2021. The earliest it could occur (two sigma) is Q2 2020, and the latest is Q4 2021.
*CIPA categories lump the Voigtlander Bessa and some Hasselblad and Fuji MF into Non-Reflex, even though it doesn't quite fit the definition of Mirrorless as people understand it, and the Reflex still picks up the Nikon F6 and FM10, and Pentax MF. These represent less than 1% or so of total volume though. CIPA also does not include Leica as they're German.
**To clarify what I meant by inelastic demand in my previous post, it's simply that users of certain models will buy the camera whether or not it's on sale. The D5, D850, Sony DSLRs, and Penhough.