The adapter isn't a flop at all. But the native glass allows better images than EF lenses. They are sharper and allow more detail. They also have that programmable ring (although isn't there another adapter allowing you to add that to EF lenses? I forget now)
The new mount allows for better glass. But better glasses isn't available yet. As I mentioned, it needs to mature.
I believe that DSLRs as they stand now, will slowly be phased out. I think the 5D Mk5 will be an incremental update to the Mk4, but the EOS R Mk2 will be a huge update over what we have now. Also more lenses will be available, and people can and will start to switch at that point. I'll be surprised if we ever see a 5D Mk6
Canon & Nikon cannot compete at this time, as they don't have the native glass to back it up. So they have saved money by making complimentary bodies to their existing line up whilst the glass gets developed and released, and THEN when thats out, they'll start picking fights with the Sony.
Thats how I think it will go anyway