The attraction of the 7D series was also the extra shooting range for motorsports, sports, airshows, birders etc. Also the high frame rate for capturing action.
With some of the RF rumours promising much longer reach lenses, the need for smaller sensors to get the reach goes away if the lenses are not too expensive.
And keeping the MegaPixels low means todays achievable frame rates can easily rise to capture the action.
But the lenses
are more expensive at desired angles of view needed for field sports and apertures needed for artificial lighting.
Compare the EF 200mm f/2.8 on an APS-C camera to the EF 300mm f/2.8 on a FF camera: $750 vs. $6100.
Compare the EF 70-200mm f/2.8 L IS II/III to the current Sigma 120-300mm f/2.8 on a FF camera: $2100 vs. $3600.
Or compare the EF 300mm f/2.8 L IS II on an APS-C camera to the 400mm f/2.8 L IS II on a FF camera: $6100vs $10000
And there's no 450/500mm f/2.8 for any price.
The attraction of the APS-C 7D Mark II (or Nikon D500) was you got all of the following: pixel density needed for cropping, high frame rate, and better reach in a highly ruggedized body that wasn't matched by any FF camera other than the 1D series.
7D Mark II + EF 70-200mm f/2.8 L IS II cost (list over the past 3-4 years) $1700 + $2300 = $4000
1D X Mark II + EF 300mm f/2.8 cost (list over the past 3-4 years) $5500 + $6100 = $11600
None of Canon's 5 series bodies to date match the 10 fps of the 7D Mark II.
None of Canon's 5 series bodies to date match the 200,000 shutter rating of the 7D Mark II.
Even with the 30 MP 5D Mark IV, cropping to APS-C size leaves less than 12 MP.
At the time he tore down a 7D Mark II in early 2015, Roger Cicala said the 7D Mark II was the most weather sealed camera he had ever been inside of. The 1D X and 5D Mark III had both been on the market for almost three years at that point, so presumably Uncle Roger had seen their innards by then.
I think the reason both Canon and Nikon abandoned the concept was not because there is not enough market demand for them. It's because they realized how much camera they were selling for one-fourth the price of their FF sports bodies and also how much they assumed they were losing selling 70-200mm f/2.8 lenses instead of 300mm or 400mm f/2.8 lenses. What they fail to account for is that many 7D Mark II/D500 shooters using 70-200mm f/2.8 lenses wouldn't buy FF sports cameras and 300mm f/2.8 or 400mm f/2.8 lenses even if the APS-C option is not available. They either can't afford it, or aren't willing to pay that much for it.