I think it is mostly the very popular stuff that gets re-uploaded. I am guessing that the re-oploads only comprise a relatively small fraction of the actual 400 hours a minut, worth of uploads. But lets say re-uploads were 50% of all uploads. 200 hours a minut, is still staggering and that is on youtube alone.
You mention droneshots as being popular. The DJI Maveric drone is actually capable of 120 frames/s in 1080p, so they thought it was important enough to incorporate. The Iphone has been capable of 240 frames/s since version 6, and I see a lot of those super slo-mo videos on instagram, from people my age and younger. I call that pretty casual use of slo-mo.
Now I know High frame rates are a lot harder to implement on larger, high megapixel sensors. But to say that the utility of slo-mo is to niche to prioritise in a high end DSLR or Mirrorles , really clashes with my experience as a creator and a consumer of media content.
But I guess we will never know, less we conduct a thorough study