The 5Ds and 5Ds R had a few features the 5D Mark III did not, though it is arguable if some of them made much of a difference for the intended use cases of the 50 MP bodies.
Flicker reduction and an RGB+IR light meter are the first two that come to mind.
Then barely a year later the 5D Mark IV came along with all of the improvements of the 5Ds over the 5D Mark III, plus a few more.
By the time this 100MP+ camera is available, the R5 Mark II might be barely a year away with somewhere around 50-60MP.
There may have been more than a few folks who bought the 5Ds that didn't really want/need 50MP but did want/need more than the 22MP 5D Mark III who wished they had waited another year for the 30MP 5D Mark IV.
In the past, with FSI sensors, as your #pixels get too high you end up with all your sensor being non-pixel sensing support circuitry.
But new BSI has the potential that the full sensor well depth is always there, no matter how tiny the pixels being sensed. So I could see BSI technology allowing much higher resolutions while maintaining a similar IQ, which is a win in potential resolution at the cost of only much higher data storage requirements.
This almost begs the question of: If they introduce the same R5 successor in a 50-60MP version and a 100+MP version at the same price, which would you want? Most would opt for 100+ just assuming it's the best choice. I might opt for the 50-60MP sensor at the moment taking storage costs into account, but would probably wait to see some test results before knowing which was better for my use cases.
But either way, I'm eagerly waiting for the R5 style successor!