I had the same reaction as Taylor, since Helmut referred to the singleton proposal as "pointless" in the thread on "on". Then I stopped to wonder if there was a mixup. "Point-free" style, which cons and singleton both facilitate, is sometimes called "pointless" style, but usually only as a joke. It's wordplay.
Since Helmut writes perfectly good English, I had assumed the wordplay was intentional, which made me suspicious of the nature of this proposal. But from over here on my little digital device, it is hard to know for sure.
Either way, I think the plan to make Data.List qualified-by-default is great, and having a uniform abstraction over list-like things is also good. I look forward to seeing more use of backpack to take advantage of such abstractions.
I suggest that the haddocks for Data.List.cons clearly state that it is just a synonym for the list constructor, so we get a built-in "teachable moment".