In this case, owners of 7D might be unhappy about downgrading from 1.6x crop to 1.3x crop. At best, this would require buying a 1.4x teleconverter ($300 for the mark II, $470 for the mark III). At worst this might require buying new telephoto lenses, and that's really big money.
They have already done this to the 1.3 owners by moving to the ff 1DX - of which a lot were already using the 800 f/5.6. By taking away the f/8 capability there is no way of getting back the extra reach. The longest they can get now is the 600 f/4 + 1.4 to give 840.
The only way to get the extra reach would be by going to a 1.6 - which then hits the noise issues when trying to get the shutter speeds to 1/1500.
A lose/lose situation
The only way I see to resolve the issue is to continue making both both APS-C and APS-H body (whether those are called 7D and 1D or not), anything else taking something away from current owners.
[OK, Canon might be bought by santa, who would offer 1D owners to replace their telephotos with 1.3x longer lenses, but that's not a likely scenario.]
Perhaps we might copy Nikon and have compatibility modes? I dont know if it would be possible to mount ef-s and ef lens on a ff body
It's not. EF-S lenses go too deep into the mount and the FF mirror would hit the back of the lens.